Glad you asked as this as I wrote part of my PhD thesis on this!
It’s quite simple really… the main part of our skin is the dermis and it is like a composite material that is strong but stretchy; ‘collagen’ provides the strength and ‘elastin’ provides the stretch.
But elastin (just like an elastic band) eventually stops recoiling and get’s less stretchy causing the skin to sag, this happens in areas where there has been high wear. We use our face all day, every day so small wrinkles begin to form.
There are ways of slowing this process down mostly by reducing exposure to UV light (don’t sunbathe too long) and moisturise daily!
I’ve got nothing to add to what Alex said – he is the expert after all – but I would say that this obsession with looking forever young is just getting out of hand. Accept the wrinkles and grow old with style! (Although this isn’t exactly something that you or I have to worry about for the time being) Oh, and make sure the wrinkles appear because you smiled too much, not because you were frowning all the time 🙂