Basically, there are two ways you show signs of aging. (1) By naturally getting older which shows in wrinkles and etc. There are ways to slow this process (by eating well, taking your vitamins, antioxidants, not overdoing the sun, and etc), but for the most part this is a inevitable process that is out of your hands. (2) By doing things, purposeful or not, that change the way you looked at your most fruitful – we will assume this is at or around age 18.
Usually as one grows older, over time, they change their hair, needlessly trim their eyebrows too much, slim in the face area, or puff in the face area, sun damage, yellowing teeth, start doing their makeup different, facial hair (more or less of it), body weight and composition change, etc, etc. Number one you can’t control unless you have a time machine. Number two you can. When someone from the long lost past sees you and you look the same, they think, “wow he/she looks exactly the same, I want to know their secret”. The same means young to them and anyone else keeping track.
It takes work to stay the same no doubt. As you age, if your body conscious, then you try and stay lean. In doing so, your face slowly looses fat, which is hard to get back, which can make you look prematurely old. You may think, that baby fat in your face was a bad thing, but it was not. That baby fat makes your skin tight and youthful; hides and prevents wrinkles. Or maybe you went the other way and gained 40lbs since your highschool days.
In short, pull out a picture of yourself when you were 18 (or whenever you were at your most youthful) and strive to achieve that look again. That is the secret of a youthful appearance – nothing more!