The Seventh Doctor looked somewhat older in the '96 movie than he had at the end of the original series.
I think that, barring accidents, a single Time Lord incarnation would age slowly but progressively over the course of maybe 400-500 years (since the Doctor was reputedly around 450 when he had his first regeneration), so a total TL life expectancy would typically be around 5200-6500 years, each incarnation growing to old age and then resetting to somewhere between young adulthood and middle age. The Doctor, however, is an exception to the rule because he leads such a dangerous life; thus his individual incarnations have comparatively short lifespans compared to a typical Time Lord ensconced in the hallowed, safe halls of Gallifrey. His first incarnation was the only one that "died" of old age; his other regenerations were all the result of violence or other factors (enforced punishment, radiation poisoning, a fall from a height, spectrox poisoning, a TARDIS crash, surgical error, unspecified war injuries (?), vortex energy overdose, and radiation poisoning again). So his incarnations don't show their age because they don't last long enough to do so.
Although this notion is somewhat undermined by the fact that Eleven had a 200-year age jump in-story and didn't change in appearance at all.