If you go by all the on-screen mentions of the Doctor's age over the years it's actually been pretty consistent until now:
Patrick Troughton claimed to be 450 in 'Tomb Of The Cybermen'.
Jon Pertwee only ever made a couple of vague references to being thousands of year old, though these are ambiguous in the way he phrases them, i.e. "I am a scientist, and I have been for several thousand..." before he cuts off, which could just mean he's gained scientific knowledge across a span of millenia.
Tom Baker's age is mentioned several times, usually a number between 750 and 760.
Colin Baker claims to be over 900.
And it was Sylvester McCoy who claimed to be 953, in 'Time And The Rani'.
Hartnell, Davison and McGann never mentioned their ages, though in the books McGann's Doctor is supposed to be over 1000.
But then in 'Aliens Of London' Eccleston's Doctor is suddenly back to being 900 again, although in 'The Empty Child' he mentions "900 years of phone box travel", which could mean that it's been that long since he first left Gallifrey in the TARDIS.
So it seems the Doctor is getting on a bit and starting to lie about his age.