I am curious as to whether or not the Doctor is generally supposed to age. Obviously the actors that play the character do, but is the actual character's appearance supposed to be frozen in time until the next regeneration?
It seems clear that the Doctor (and Time Lords in general) age at some rate prior to their first regeneration. Thus the first doctor went from being a child born on Gallifrey to a relatively old man bouncing around the universe in the TARDIS. One wonders though if pre-regenration Time Lords experience a fairly normal (if somewhat extended) humanoid lifespan?
The question then becomes do regenerated bodies age or are they meant to be relatively frozen. This becomes more noticeable with Eleven since his story seems to cover a relatively long period time without any noticeable ageing ( i.e. he goes from around 900 to 1200 with no change in appearance).
One of the things I suggest to a friend was that you could justify the fact that the reason each of the regenerated Doctors seemed to get younger is that as a Time Lord approaches their 13th (and presumeably final) regeneration, the process extends life by giving more youth in the hopes of running out the clock. One might event suggest that the 13th regeneration would completely reset the Doctor's proverbial clock and he comes back as a baby in the hopes of having a long but relatively normal final lifespan. Thus a Time Lords life cycle would come full circle.