Quote Originally Posted by Rater202 View Post
I believe that an episode of New Who has a one-off line that says that the Doctor doesn't care about his actual age: Whenever he forgets how old he is he just starts counting from whatever random number he decides he likes at the time.
Day off the Doctor suggests that isn't exactly true with the Doctors using their ages as if they're meaningful. The best expansion I've seen is that somewhere the Doctor switched how he's tracking his age to a longer year, which makes sense.

Assuming that One had an average lifespan for a Time Lord body then a regeneration diez to old age happens around the 500 year mark (assuming that Two didn't lie about his age). That would make Seven and Right least less time than that whole War gets in a good couple of centuries, leading to him being somewhere around 1600 in the old system. War is the most likely to switch away from counting in Galifreyan years, and it's likely that he moved to a system where years are twice as long. This would make Nine and Ten regenerate at around the 1700 mark, while Eleven does so at roughly 2200(roughly the age Twelve says he is, maybe he switched back). This does assume that Trenzalore has shorter years to make it work.

But yes, the Doctor almost certainly doesn't know exactly how old they are. They can probably make a good stab at it though, and the TARDIS probably has an exact time since they stole her. Although, admittedly, nothing says that the ages he brings up are ever his actual age (except for that one time Romana called him out on getting it wrong*).

* Which could also be explained by Season 6B.