I am only 21...but I have noticed some subtle things I would like to prevent from advancing.
But it's still good thinking to start doing something as soon as possible
Mainly, "nasolabial folds" I believe are starting (there's sort of lines there)- in the past couple of years.
Is there any product you could use ? Probably not, but you could nevertheless be able to do something about it.
Some general tips : if you smoke, know that this will only make things worse, skin wise. So avoid smoking. Having a healthy diet will help too. Getting enough sleep as well. You may have noticed it, but you'll look worse when it's late, and better just after waking up - I'm tinking about your nasolabial folds, for instance.
Now for the particulars :
Nasolabial folds - smile lines, seem to have several main causes. As you age, your skin looses collagen, becomes thinner, and your facial muscles aren't as strong as they used to be. You'll also loose fat in some places (which become gaunter), gain it back in other places yet (usually where you wouldn't). Also, repetitive mechanical strain may fold (normally permanently) your skin along some lines.
So, you could try tazarotene or tretinoin to replenish collagen (there are a few persons who seem to know a lot about this here, they'll explain better than I could);
you could try to work some of the muscles of your face, so that your flesh wouldn't droop as much as it would otherwise (which muscles exactly, I don't know yet for sure, though learning some face anatomy could answer that question);
Loosing fat : I can't think of anything but chirurgy to correct that one;
Mechanical strain : avoid stressing your face. Some people go as far as saying they wouldn't ever (or rarely) smile, lest they worsen their smile lines. But even barring that, you could try not to frown or pout or any other facial expression that you'd think could worsen your predicament. In other words, keep a peaceful, relaxed face in most situations.
Besides all this, there's mor yet I think, first you could worry about double chin, sagging jowls, allopecia, drooping-weary eyelids, gaunt cheeks, shaving and the permanent undergrowth of stubble you'll get as a result, upturning of your lips (from concave to convex), thinning of your lips, the luster of your hair, the white of your eyes, the fact some of your face's features (bones) will carry on growing, changing the shape of your face, etc.
Well I'll try not to get too much over myself as that's a topic I've been interested in. Good luck anyway !