How do Time Lords age in Doctor Who?

More specifically: why did Matt Smith’s Doctor look so young on his ‘farewell tour’ (the 200 years he lives through in series 6), but become an old man while defending the town of Christmas on Trenzalore for 300 years in The Time of the Doctor?

Put simply: why did he age like George Clooney in the first case, but Macaulay Culkin in the second?

https://www.youtube.com/watch?v=6AxCoMbiZfk

Well, thanks to a theory posed by Redditor crawlspace_taste, we many finally have an answer…

Time Lords only age when they remain in one time

And conversely, they don’t age while travelling through time thanks to exposure to the time vortex.

The evidence? Well, before pinching his Tardis, the first doctor spent most of his life on Gallifrey, hence his aged appearance. No time machine, no eternally youthful face.

And the 11^th Doctor doesn’t appear to age through series 6 because he’s travelling in the Tardis. However he does look way beyond pension age in The Time of the Doctor as he’s without a time machine.

It’s an explanation that not only fits logically, but has plenty of artistic merit. It suits suits the Doctor’s nomadic nature: keep swimming in the vortex of time or die (albeit slowly).

It’s also a theory that explains why The Curator in The Day of the Doctor – the one wearing the face of Tom Baker’s fourth doctor – looks so much older than the scarf-wearing doctor we saw in classic Who: that time lord has retired, stayed on earth and thus aged. A lot.

https://www.youtube.com/watch?v=1cbQVfVfSrI

Fortunately, you've only got to age another month to see the next series of Doctor Who (unless you're reading this from the Tardis, that is).