I must confess I have the Doctor on the brain. Likely because I have been watching a selection of old episodes in preparation for the 50th Anniversary Special to air on November 23. I have watched the first 9 Doctors and it has led to what I think is the answer to a question: How old is the Doctor.
We are only given his age from the perspective of an outsider one. Romana, when she first came aboard the Tardis, corrected the doctor on his age saying that he was 759 and had been traveling in the Tardis for 523 years, placing his age at 236 when he "borrowed" the Tardis (now shown on screen toward the end of the 7th season of the new series). Outside of that we have to rely on the Doctor's own measurements of the passage of time.
I'm not about to get into whether the Doctor is talking about human years or Gallifreyan years. I'm assuming that since we watch the Doctor in English that everything is in Earth years (and converted from whatever other measurement might be used to Earth years by the Tardis translation circuit if you want to go that far). But that one scene in the Rios Operation gives us a good clue about how accurate the Doctor is about his age. He isn't. He openly lies about it all the time.
That leads us to an estimate of his age at each point in time. 236 when he "borrowed" the Tardis. About 450 at the time of his first regeneration. We have to guess at the next one, but I'd say about 600 at the time of his second regeneration. About 750 for his third regeneration (which jives with what Romana said). But just over 800 at the time of his fourth regeneration. By the time of his fifth regeneration, he was 900. We are given an age of 953 for his sixth regeneration. The accuracy of that number is questionable. His next incarnation was over 1000. And that is where the old series ends and we appear to have a bit of an issue.
But it is a phantom issue. In the new series, the Doctor gives his age as 900 frequently. But that 900 has a source and the Doctor himself gave it to us. It isn't his age, it is how long he's been traveling in the Tardis. We of course have no estimate of how long between when he left Gallifrey in the Tardis to when Ian and Barbara joined him and it got stuck in the familiar form of a Police Box, but it wasn't long enough for Susan to age much (provided we even had a clue how old she was in An Unearthly Child or when she left Gallifrey). But the Ninth Doctor specifically says 900 years of phone-box travel which means he has lopped off the time before that, or 236 years. This makes the Ninth Doctor about 1136.
The Tenth Doctor kept his age in the 900's so we have no idea how much time might have passed. While that fits with his earlier age before he regenerated, it doesn't give us a clue to how long this incarnation lasted. He went out saying 906 (adjusted to 1142 to account for when he left Gallifrey). The Eleventh Doctor started off at 907 and claimed up to 1103 (1339 adjusted for when he left Gallifrey).
The Doctor leads a dangerous life. More so than most timelords. His first regeneration happened naturally and from that we can guess that timelord life expectancy is something like 6000 years. Yet the Rani was nearing 1000 and had not regenerated and the Master was done with his 13 lives by the time the Doctor was 750. If the Rani is more typical (and there is nothing to indicate she is) then a timelord should live for 13,000 years. But given Borusa has a different regeneration every time we see him, I would say that it is very variable.
Given the Doctor's tendency to lie, estimate, and reset his age, nothing he says about it is accurate. Another thing the we have to question is how his companions age as they travel with him. Do they age normally or slowly. Is part of his lying to avoid telling his companions that they have been with him for ten years but only aged one. Plus the Doctor has periods when he is alone. How well does he keep track then. For a TV series that has such a fundamental mystery at its core (who is the Doctor), it really isn't surprising that we have no answers for most of these things.
I think it is safe to say that the Doctor never has said he is older then he is (not without some ulterior motive) so as we wind down the Eleventh Doctor's time and look to the Twelfth, we can be assured that he has shaved at least 300 years off his age when he told Clara he was 1000. With all the things he has done off camera, I think it is safe to place the best estimate for his age at about 1500 as he goes through his eleventh regeneration this Christmas. That's assuming there aren't a lot more side trips unaccounted for.
