For a lot of women, a side effect of ageing is that their lashes get thinner and sparser - and their immediate reaction is to switch to a volumising mascara.
This is a mistake. As your lashes get thinner, they also get shorter, and a thickening mascara can give you a very clumpy look, like a teenager who's just learning. Far better is to switch to a lengthening mascara, with very very fine fibres in it, and apply two or three coats as needed. As with all mascara, open your eye wide and apply the product to the roots, then wiggle the brush up through your lashes. This way you get a natural look, and a strong eyeline. For daytime, stick to brown mascara unless you have jet-black hair, as anything else tends to look too harsh.
Personally I favour the Respectissime dual-end one by La Roche Posay, which has a conditioning wax at one end and the mascara at the other. The wax makes the mascara glide on really easily and stay put once it's there. Other manufacturers offer similar products.
For daytime, however, I recently also bought to their Definition mascara, which has a very fine wand and is perfect for building up exactly the coating you need without either lengthening or volumising your lashes. It's great if you have sensitive eyes, as it's fibre-free and hypoallergenic. In terms of how it looks once it's on, it reminds me of the old cake mascara from the 1960s, that you had to wet and then applied with a little stiff brush. This was time-consuming and fiddly, but the end result was very controllable and built up smoothly for a perfect result. Like that old kind of mascara, Definition is totally water soluble, so will streak your face at the slightest teardrop, but my eyes are so tender that I prefer this to a waterproof one.
Your 40s is also the time to think about false eyelashes for special occasions, and to practise in the bathroom (not five minutes before your wedding ceremony). The newer false lashes are a completely different animal from the spidery wings we all knew growing up (which went with purple, blue or green eyeshadow and frosted lipstick). They come in strips of varying lengths, look very natural, and you should just cut a few and apply them to the outer corner of your eye. Then curl your lashes as normal, then apply mascara as normal, and the whole lot should blend in nicely. For an idea of what's available, check out Ardell.
There are, of course, even newer options on the market, such as lash extensions, which are woven into your existing eyelashes just like hair extensions on your head. This, I feel, is going too far down the route of decadence at $300 a time (plus endless top-ups that cost nearly $100), but a woman's money is her own to spend after all. The results are spectacular and you can see them here. Once you have eyelash extensions, you can't use waterproof mascara or oil-based make-up removers, or you'll bring the bonded lashes off, and there is one other downside - some doctors believe their overuse can lead to you losing your eyelashes altogether, a condition known as traction alopecia.
For women who have lost their eyelashes altogether - a situation that is becoming sadly more common due to chemotherapy - you can have an eyelash transplant. It isn't cheap, coming in at about $6,000 and currently performed only by one doctor in Florida (though he is training others), but the results are permanent. The transplants aren't eyelash hair - they're normal hair taken from your own head, so you have to trim them, dye them and curl them in order for them to look like normal lashes. Only you can decide if this is worth it (and by the way, the surgery's performed under local, not general anaesthetic). For details, check out this YouTube video.