Spoiler
^^^Supporting evidence^^^ Don't worry, all pre series 8 stuff.
So, what if the Doctor's ageing process varies from body to body, because of metabolism or whatever. Even the eighth Doctor said his particular body was good at healing itself.
What if the Doctor can use residual regenerative energy to youthen himself. Keep himself looking spick and spam. Purely from a cosmetic stand point.
This way we can retroactively insert eras of the Doctor's life where he lived as an old man for a while before giving himself a top up, and old actors can come back and play the part without the difference in appearance being too jarring.
The risk here is that it cheapens the change in regeneration. But I think as long as its made clear that this is only for minor injuries and cosmetic rejuvenation, it'll be fine.
Like the Doctor can return to his companion one day, a bit older, and he almost off-handedly does it. Rubs his hands together building up a glow "lets see if I have enough residual energy to..." swipes hands across face, young again, rubs a little more down his arms as if dusting his shirt, the flings the rest down his body, "There we are."
I'd like to see some of the older Doctors make a come back, either in anniversary specials or cross over episodes like "The Two Doctors". I think this would be a good way-around y'know.
I mean, the alternative is that they come back anyway, and we just don't question their looks.
Thoughts?
PM can easily come back, DT would do it! Matts already said all they have to do is ask!