>It might be interesting if someone were to ask him sometime, because
>when you get right down to it, he doesn't really know exactly
>how old he is any longer. He reckons his age against the anniversary
>of his birth on the timeline he's native to, so that, for instance (as
>we saw in Night of the Kissogram!), he marked his 438th
>birthday in June of 2411 - but given the cumulative effects of both
>time and dimensional travel, that's only remotely accurate.
>
>For example, he spent longer in the Split Infinitive universe
>than he was gone for, and one expects he was not missing from his
>"regular" life for the better part of a year despite having spent that
>long in 1943. He's closer to 500 - probably quite a bit closer - but
>he himself has only a vague idea how much closer. If he sat
>down with a legal pad and a Sharpie for a little while he could
>probably work it out to within a month or so, but he could never be
>exact.

So basically he's got the same problem as The Doctor, having spent so much time traveling through time and space that the best he can do is guesstimate his actual age.

"How old are you now?"
"Uh, I don't know, I lose track. Twelve hundred and something, I think, unless I'm lying. I can't remember if I'm lying about my age, that's how old I am."