Why the Doctor cannot be in his 900s

[VerticallySidewards]

lol who knows  

[Doctor the Sixth]

This is the truth:

The Doctor is

 

[123WAFFLES]

Well maybe it has something to do with the fact that HE'S A TIME TRAVELER!!!! Can you imagine how hard it would be for a time traveler to keep track of his age??? No, his age doesn't change when he travels, but still. Just something to think about...  

[Aaya-San]

Maybe it's not "legit," but on the special edition of The Weakest Link featuring all Doctor Who cast, David claims to be, playing as the Doctor, 925 years old.

You can take that as the age or you can think he's just throwing around numbers.

Also, there are adventures in between the show so the age can change there(cough books are half awesome cough).

I figure he usually just throws a number around because even Davies would notice the flaw.  

[Deus Phoenix]

Actually Steven moffat has confirmed the doctor dosent remember his age, and is possible he has mixed up his age with for the amount of time he has travelled.  

[shukuo]

I'm of the camp that when you have nearly a thousand years of life behind you, then the exact number can get a little fuzzy. It's not like he can say, "Well, I was born in X, so therefore this year, Y, I'm Y-X years old" the way we do it.  

[silversonic1]

Well, there are several possible reasons that the Doctor says different ages. For starters, he lives inside the TARDIS. Unless you set up a system of keeping time inside a time machine, you can't keep track of your own life.
The second reason is the writers switch around so much that it's hard to keep track of a little thing like age. The third is likely because he may have originally gone by Gallifrey years and not Earth years. It's possible that Gallifrey has longer years, thus after conversion, makes him seem older. The fourth possibility is that during the break between the end of the classic run and the beginning of the new run, he spent many years traveling alone, as was hinted in the first episode. How many years, however, is unknown. And the final reason is because the Doctor likes to be impressive. He lies, and occasionally rounds up. Even when his adventures don't actually add up to a year, he tends to add one if a year's passed in the present('06 to '07 equals +1, for example).
Needless to say, his age is a mystery. Too much thinking about it would even give a Time Lord a headache.