It's arguable that, since the Time War eliminated all other Time Lords, and conferred unlimited regenerations upon The Doctor, that the regeneration process itself varies in relationship to the number of total Time Lords in existence.
If that's the case, then effectively what we see as regeneration is really "taking another dip in the Time Lord Life Pool" shared amongst them. In a universe with many Time Lords, it would make sense that one would regenerate "older", taking a smaller sip from the fountain of youth… With fewer (or no other) Time Lords, The Doctor's regenerations have a more powerful effect, granting him a more youthful appearance.
Aspects of this possible answer appear in the episode "The Christmas Invasion", in which the newly-regenerated 10th Doctor (played by David Tennant) loses his hand in a sword fight, only to have it grow back in moments later!
Outside the Whoniverse, of course, the answer matches much more closely to the variance in age and appearance of James Bond – that which will appease the perceived market/viewership the most. In the case of David Tennant, at least, they were exactly right.