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Also, what was up with the Doctor aging so much in 300 years in the Christmas episode? Wasn't there 400 between the start and end of the last season?
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Notice how people don't age until later in their lives? They grow up, sure, but they don't start to get wrinkles until their late thirties. That's when their cell's regeneration cycles start to run out and they are no longer able to maintain their bodies near-perfectly the way healthy kids, teenagers and twenty-somethings are. Mitosis in multicellular organisms is a lossy format of cellular reproduction, so there are telomeres on the ends of the chromosomes which shorten each time. When the run out, it eats into the genetic code, the cell lineage falters and comes to an end, and the organism they belong to begins to show visible signs of ageing. One of the few ways to renew telomeres and thus the regeneration cycle is to recombine the DNA, which is how children are born (and why clones come with a health risk), and apparently also how Time Lords regenerate.
My suggestion is that Eleven was never going to visible age until those last few centuries in that body (fighting those incursions probably didn't help either, War certainly showed his age after spending his whole regeneration in combat). Ten also aged the same way when the Master forced those centuries on him.
It makes sense to me, though I only award those marks to the writers if they thought of that themselves. They shouldn't be depending in me to make their work make sense!