We've seen him have so many adventures (with who knows how many unseen ones in between), and he's had five or so regenerations - all in a time in which he's somewhat consistently claimed to be 900 (or, in a few cases, 1000ish). So if he *does* know what he's talking about regarding his age, all we've seen of him since the late 1970s/early 80s or so happened, for him, in just one century - at *most*. And quite possibly in as little as a few decades. There were much greater age gaps between his earlier incarnations, but nowadays he seems to be dying excessively often, without aging appreciably in between.