As we know the Doctor, regardless of his total age, has the ability to regenerate into the form of a body seemingly older or younger (in terms of physical appearance) than the previous one (Doctors such as the 11th, 10th and 5th started out looking rather "young", while other incarnations such as the 12th and 3rd began with their bodies appearing "older" from the start).
The 11th Doctor's 2013 Christmas special The Time of the Doctor story made it clear that if one incarnation lives long enough without regeneration, the body will physically age (I'm aware this is either demonstrated or implied in earlier stories, but I'm just going with the most obvious example); Eleven can be seen transitioning from the appearance of a young man to that of a much older man towards the end of the episode, but what if the 12th or the 3rd were in that scenario? Would they have died of old age much sooner than Eleven, as they had older bodies from the start, or would something else have happened?