The executive producer of Dr. Who states that the doctor didn't know his real age (implying when he does state an age its a guess or a lie).
"The thing I keep banging on about is that he doesn't know what age he is. He's lying. How could he know, unless he's marking it on a wall? He could be 8,000 years old, he could be a million. He has no clue. The calendar will give him no clues."
In retrospect this makes a lot of sense. When your ona different planet at a different point in time every other day you have no real reference point to judge your own age by. Eventually you simply have no idea if you 100, 1000 or a million.. Days and years on planets are all different and you never stay for long enough to count it...
@a1batross well i dont think its so much about a matter of lack of objective units by which to measure. Its more about bothered to actually do it.. Unless they wear a watch and and keep a reference time in seconds (which Dr. Who doesnt appear to do) having an objective measure he could use doesnt really help him much.