Or I wonder if it's a conscious thing that a TL would do? Like 11 didn't age visibly because he didn't want to look older, perhaps that's why River was always so worried about him using regeneration energy, he was using it to keep himself younger looking. I mean 10 once used some of his energy to help restart the TARDIS in the parallel universe when they first met those new Cybermen. "I just took ten years off my life"
What would be interesting through - is if the 13th regen was exactly that, he comes back as an infant and has to grow up all over again. And lives one final very very long life - providing he doesn't need to regenerate. Although then again that's really only 2 more Doctors so either they simply ignore the 13 regen's rule or they simply say that the 14th regen is a total reboot to the Doctor, maybe TL's lose a lot of their old memories. Perhaps his memories only go back to his time as the 2nd Doctor. His entire first 'life' is forgotten. 15th, only goes back to the 3rd, 16th only to the 4th and so on