People who've maintained a fit, healthy weight throughout their life will often have a younger looking physique, as well as less sagging in the skin in their jaw and neck.
Speaking of jaw and neck, those who've maintained a full set of teeth also avoid the bone loss in the mouth and jaw structures, which gives the face a sunken appearance. This is a big reason why the Faces of Meth is so shocking. These young people lose their teeth and they start looking old.
Good teeth (whether real or enhanced) can also make a person appear younger. Teeth naturally darken with age; the gums recede and the lower teeth tend to get crowded around the age of 35.
Vibrant hair color, with a glossy texture and lots of volume can also make someone look younger. Again, this can be real or enhanced, though often one has to pay a price for having nice looking hair at a certain age. (Though this is one of the easiest fixes; many older female celebrities, such as Hillary Clinton or Barbara Walters, have beautiful hair. It doesn't have to look natural as in non-color treated, it just needs to look nice.)
Modern, stylish clothing, eyeglasses, jewelry and accessories can go a LONG way toward appearing younger. The cut of the clothing is particularly important. Lots of people think, "well, classic Levi's don't go out of style," while ignoring the fact that their high-waisted, pegged cut screams 1994.
In some cases, having certain racial characteristics tends to skew age perception. In the U.S., blacks and Asians (or, mixed race people with black and Asian ancestry) are often thought of as looking younger than Caucasians.
Then, there's also the way the person carries themselves and makes their way in the world. Age guessers at fairs will often look for these cues: what slang the person uses, how they move their body, who they're with, their voice quality. Everyone knows someone who seems older, or younger, than their peers based on these subtle "tells."