It's strange when you realise you used to have a disease but never realised.

Well, not a disease exactly - more of a 'complaint'. It's called trichotillomania, and it means pulling your hair out. 

I used to do this all the time when I was about 11-13 and I have a permanently high forehead because of it. Lucky not to end up bald.  Apparently a lot of women do.

From what I was reading in the Guardian article on the subject this morning, the tearing out of the hair has an element of ritual - one woman described rolling the hair between her fingers in satisfaction. Personally I used to eat mine. And as for others, my eyelashes were a prime target. Again luckily for me, mine grew back again.

Like a parrot pulling its feathers out, isn't it? And obviously a bad sign - it's interesting that nobody noticed. I got told off for it being 'dirty' but nobody ever wondered quite why I might be doing this. But then nobody ever wondered why I decapitated my dolls and drew blood on them, and hanged my teddies off the washing line when I was little. Or why my brother spent all his time designing fireworks. Or why I constantly mixed up jars of congo red and stood them round the room like blood. All seemed quite normal at the time...Today I'd probably be sectioned.

Apart from the fact that the social services in the 1970s were just as crap and overworked as they are now, it takes quite a lot to put two and two together. For instance, even as an adult I'd never connected my teenage anorexia with my mother's habit of smacking me once across the mouth while I ate. It took my friend Malc to say: "Good way to give someone an eating disorder..." before the penny dropped. Doh.

Oh la. Well, at least I don't eat my own hair any more. And at least there's help for people who do.

I was watching Professor Regan's pharmacy last night, having followed this series with some interest.

For those who don't know, Prof Regan is a scientist who takes issue with products that make scientific claims that they can't back up. Last night's programme threw up some interesting issues about ageing. 

Although Prof Regan didn't deny that fillers such as Restylane are very effective and do exactly what they say on the tin - fill in wrinkles - she also discovered that is is not necessarily your wrinkles that actually age you. 

She did this by going to a face recognition centre, where she was asked to guess the ages of numerous faces flashed up in front of her. They went past at high speed - in fact, said the scientist conducting the experiment, most of us can 'age' someone in around a tenth of a second (talk about first impressions....). 

Prof Regan guessed ages correctly within 2.8 years, and this is apparently typical. But what was interesting was that she could do this - as we all can - EVEN WHEN THE IMAGES WERE BLURRED. In the blurred pictures, wrinkles weren't visible so clearly, we must all of us judge age on more than wrinkles alone. 

He then showed her that there were two kinds of images - some had shown the full face, with hair and ears, while some had been close-cropped vignettes, showing only the centre part of the face, with no hair visible. With the vignette images, her accuracy rate had dropped to 3.8 years. 

Judging from this that hair might be more important than wrinkles in terms of guessing someone's age, Prof Regan then did another experiment. She took a variety of mothers and daughters and got them to swap hairstyles, with the help of some wig artists who duplicated the looks that the women were wearing. No other changes were made to clothes or makeup, and groups of subjects were then asked to guess the women's age.

Overall, the subjects put the daughters (now wearing their mothers' hairstyles) at three years older than their real age, and the mothers (now wearing their daughters' hairstyles) at four years younger than their real age.

The message of this is fairly simple - if you want to knock a few years off, get to a hairdresser, which is is a crumb of comfort for those of us who don't want, or can't afford, to go down the injectables route.  

Once you hit 40, you will probably notice that your hair starts to change. Often it becomes dryer, along with your skin, and for many women, this is when their grey hairs really start to get a grip.

If you are used to colouring your hair, it often helps to break up your colour now and switch to highlights and lowlights rather than have an all-over shade, especially if you've had an obviously artificial colour. Strong, bright, all-over artificial shades are fun when you're young, but they need a Zandra Rhodes-size personality to carry them off when you're older. Instead of looking current, it can make you look like you're trying too hard to look younger.

It also pays to go a little lighter at this time, as the pigment in your skin is starting to fade and your hair colour would naturally keep step with it if left to its own devices. One tip is to think of the colour you had as a child. If you were blonde as a child and over the years your hair has faded to brown, for instance, you can probably take added blonde shades very well. Highlights AND lowlights, especially graduated towards your face, are very flattering and can give your hair colour real depth.

At 43 I decided that my naturally fair hair had gone dark enough and was verging on mousy, so I switched back to blonde. There are five shades in there, including lowlights and highlights, with the back and the undersides darker, and the front and top layers lighter. This gives the hair more vivacity and impression of movement.

Grey hair

Western society has a problem with grey hair because we do not respect ageing, or any outward appearance of it. I remember seeing a gloriously grey Emmy-Lou Harris in the 1990s, and thinking how beautiful she was, just as my mother -in-law (fiercely blonde in her late 70s) exclaimed: "God, hasn't she got OLD!!!"

Whether or not to go grey is therefore a very personal matter. Many women choose to cover their grey hair in their 40s but allow it come through in their 50s, for instance. My view is that blondes can get away with redying to blonde shades, but that darker-haired women always look better allowing the grey to come through, especially if their eyebrows and eye colour are dark. As you can see here, Helen Mirren switches from blonde to grey and back again as feels like it, or as roles demand, while these before and after pictures of author Anne Kreamer speak volumes - going grey took years off her, compared with her heavy chestnut colour.

Grey hair doesn't usually come in all at once - some women get threads appearing all over, like Charlotte Rampling (left), while others find it appears more in certain areas, especially around the face. However, if you colour your hair, the place you'll notice the grey is your roots. If you decide to let the grey hair grow in and the fake colour grow out, switch to temporary colourants until about 60 per cent of your grey has grown in. Then get your hair cut and allow the grey to show.

A great approach when going grey is to ADD more grey than there is naturally in the form of highlights. This can look very pretty, as grey in itself is a beautiful colour with many variations - pewter, charcoal, platignum etc. Many dark-skinned, dark-eyed women look very good with lowlights added back in quite a thick section around the facial area - creating a dark frame around the face, or the obverse, with the front section of the hair made much paler than the rest, again creating a frame. Personally, I'd love to see a grey-haired woman with fine but zingingly artificial streaks like purple, pink and blue in her completely grey hair - it would be perfectly clear that you're not naffly trying to look younger with this treatment, but simply more interesting.

Salon colour

Grey hair is different in structure from pigmented hair - often it is thicker, dryer and more wiry. It also takes artificial colour differently. Therefore I would recommend salon colouring rather than at-home colorants once your hair goes grey. Your hair needs more tlc and over-the-counter options are less effective - it can be very useful to have the heat-activated deep conditioners and subtle heat-activated colour glazes that only salons offer. At home, use shampoos and conditioners specifically designed for grey hair, which will help support the colour.

Watch out for the colour of your eyebrows and consider having them lightened if they are very dark. Brow lightening can soften your whole face. And keep your brows in good trim - heavy, dark eyebrows coupled with greying hair can look surprisingly mannish, especially as your face loses its youthful bloom. If you have the odd grey, wiry eyebrow hair coming through, it often behaves differently from your normal eyebrow hairs and can't be tamed. Pluck it out or cut it off at the roots with sharp scissors.

Cut and condition

As for length, again that's a personal matter. As you can see here, Dame Judi Dench looks great with very short hair (she's also, btw, using one of my fave clothing techniques - picking up her eye colour in her accessories), but some women prefer to keep their hair longer. More important than length when it comes to grey hair is the condition. Whether you colour, or don't colour, once you're grey, eat lots of protein to keep your hair in good fettle, and give your locks a hot-oil treatment once a week - you need to condition the bejasus out of your hair now to keep it looking good. And don't smoke! Grey hair picks up cigarette smoke and changes to an unflattering brassy yellow.

Grey hair and coloured hair may take more time and attention than naturally pigmented hair, but it is worth the effort - get your hair right and it's like putting a jewel into a great setting.

As you may have noticed, hair fascists tend to go one of two ways with hair over 40. The first suggestion (presumably written by 20-year-olds) is that you cover all the bits of your face that don't measure up to some invisible standard - crows' feet, forehead wrinkles, steadily growing ears, etc. If you follow this prescription, of course, you might as well wear a yashmak.

The other suggestion is that you should have it short, so that it doesn't suffer from weakness and breakage and is easy to maintain in your busy schedule of being a worker, wife and mother.

Personally, though, I can't help feeling that very short hair doesn't favour most mature women either. It's hard enough to get away with it in your 20s. Short cuts leave your features very exposed, which is fine if you're very thin or very cute, or have very strong features, but those aren't descriptions readily applicable to most of us, especially once we hit the big 4-0. On a striking woman, a short cut can look great, but on a heavy woman, as seen at right, a short style can look butch, especially if it shows the ears (which is a characteristic of men's cuts). Short styles also lack movement at a time in your life when you might very much want to accentuate your femininity rather than detract from it. This woman's heavy jaw and neck are fully exposed in this style - something a tad longer would be infinitely more flattering.

Longer hair can usefully draw attention away from areas you're not happy with, and add length or width to your face, depending on how it's cut. It's also more versatile to style and accessorise than short hair and you can really ring the differences between day and night. By longER, of course, I really mean somewhere between chin length and shoulder length as in this golden zone there is a length that suits most women.

The most flattering style for a mature face is generally a layered cut that ends either at chin level or just above the collarbones. The layers give your hair more body - important if your hair is starting to thin out - while the length gives movement and casts shadows on a jawline or neckline that is starting to soften. This choppier, layered, more casual look can take years off you.

Blunt cuts, even if you've worn one all your life, are best avoided. Like very plain clothing, blunt cuts create hard lines, which benefit from perfect skin and teeth to set them off. A formal blunt cut can be really ageing, as if you're wearing a hair helmet (ALL formality is ageing, which is why your look benefits from becoming more casual as you get older) - you can see this clearly in this before and after shot of Anne Kreamer, author of Going Gray (and more on grey hair tomorrow).

When you're looking for inspiration for a new hair style, don't look in hair magazines, with their extreme trends aimed at teenagers, look instead at what the women you admire are doing with their hair. This might include newsreaders, actresses and television presenters, or just people you know, or see in the street. These don't need to be women the same age as you, or even women who look like you, though that helps - you just need to identify what it is about their hair style that you like.

The change can be very liberating. A few months ago I moved from a blunt-cut bob with a fringe (left) to a more assymmetric, off-the-face look (right), partly inspired by that of CNN news presenter Sasha Herriman (below). Although Herriman is blonder, taller, thinner and 35 to my 44, we share similar attributes, such as a long nose and thin top lip, so I had an idea that this might suit me. I found, to my surprise, that the new cut not only opened my face out, it also balanced the marked assymmetry of my features, which I'd spent most of my life trying to correct with centre partings and heavy fringes. I'd clearly been barking up the wrong tree for years, and despite my husband's love of my former jet-black Louise-Brooks-style bob, he agrees that this current style is the best haircut I've ever had in terms of it actually suiting me.

Tomorrow - hair colour for the over-40s