AN O'BSERVATION ABOUT THE DOCTOR'S AGE
There's one last point I'd like to make about the two-part story of 'Doctor Who' dealing with the Slitheen.....
In a quiet moment just before all hell broke loose, the Doctor revealed to Rose that in "900 years of Time and Space, and I've never been slapped by someone's mother."
This shocks Rose, and she asks him if he is 900 years old.
Now, I would think that his previous statement would infer that he was older than that, that he had spent 900 years in traveling.
But you could tell he jumps on this figure as his age a little too eagerly, as though he didn't want to admit how old he really was. I suppose the argument could be made that he was just shrugging off the answer to avoid going into details with her.
(There has been some indication throughout the series that the Ninth Doctor is a bit disdainful of the Humans' capability to grasp certain concepts beyond their boundaries of comprehension. In general, Mickey Smith seems to be the stand-in for all of Humanity when it comes to these jibes. But there are also times when the Doctor snipes at Rose as well.)
Still I'd like to think the Doctor was insisting on being 900 years old because of vanity. (It wouldn't be the first time!)
After all, look how put out he gets whenever someone makes comments about his leather jumper, never mind his ears!
The contributors at Wikipedia, the great online encyclopedia, have worked out a rough estimate as to the Doctor's true age:
"The Doctor's age
The Doctor's age has been stated (or estimated) in several stories. In the serial The Tomb of the Cybermen the Second Doctor told Victoria that he was around 450 years old. The Second Doctor was also seen to carry around a 500-year diary in which he kept notes.
By the time of The Brain of Morbius, the Fourth Doctor was stated to be 749 years old ("something like 750 years" in the prior Pyramids of Mars). In The Ribos Operation, the first Romana said the Doctor was 759 years old and had been piloting the TARDIS for 523 years, making him 236 when he first "borrowed" it. In Revelation of the Daleks the Sixth Doctor was 900 years old, and in Time and the Rani, the Seventh Doctor's age was the same as the Rani's, namely 953.
In Remembrance of the Daleks the Seventh Doctor said that he had "900 years experience" rewiring alien equipment. In the 1996 television movie, the Eighth Doctor kept a 900-year diary in his TARDIS.
The large gap in years between the Fourth and Sixth Doctors can be partially covered by the fact that the Fourth Doctor traveled alone for a time or with an equally long-lived Time Lady as a companion, allowing for several decades or centuries of untelevised stories to take place.
There was also a gap just after The Trial of a Time Lord which can account for the Doctor's difference in ages between Revelation and Time and the Rani.
While the Fifth Doctor was never seen without a companion, there was a period where he was traveling with Nyssa of Traken, who, not being human, may not have aged normally.
How this figure [of being 900] is to be reconciled with the Doctor's age in the rest of the series and other (arguably non-canon) sources is uncertain. Possibilities include the Doctor estimating his age or lying about it out of vanity (in The Ribos Operation he gave his age at 756, although Romana insisted it was 759).
Another possibility is that the Doctor is simply referring to the years he has been traveling for simplicity's sake, which, if he began at 236, would make him 1,136 years old."*
I like the idea that he might be actually 1,1136 years old. It would then suggest that the Eight Doctor perhaps had a century or two of adventures we never saw.
(And there's no reason we can't one day go back and in Time to see these adventures with Paul McGann as the Doctor. After all, we went back in Time to see the early years of 'Star Trek' with 'Enterprise'.
At least with 'Doctor Who', it's a lot easier to play fast and loose with established History.)
Wikipedia does have one final word of caution:
"All this also presupposes that the figures given correspond to Earth years and not Gallifreyan."
* The Wikipedia entry also delves into evidence culled from "non-canon" sources like the novels. But I think I'll just give that a pass. After all, I don't focus on just 'Doctor Who', but ALL TV shows. If I incorporate spin-off novels for one, I'd have to do it for all. And I'd force the inclusion of "Ishmael", a 'Star Trek'/'Here Come The Brides' crossover story by Barbara Hambly before I'd ever consider the 'Doctor Who' books.