The actual age of the Doctor is something thats been argued for years (not least because he lies about his age a lot), and we know that he spent centuries defending Trenzalore you can easily argue that the 11th Doctor is the longest (and oldest) hes been between regenerations. Hes at pains to point out that hes two thousand years old, by the end of Deep Breath although he could be lying again. Whatever his genuine age, thats a long time to spend travelling the universe. Thats a lot of past. This season seems to be making a serious point of referencing older stories and past episodes, much more than is normal even for a Steven Moffat story. Were going to go out on a limb here this season of Doctor Who is going to be about the past catching up on the Doctor in a big way. Time will tell what that actually means for our favourite Time Lord, but it doesnt bode well for him.