So, having only watched one episode of Nine's run, most of Ten's run, and the first season of Eleven's run, I'm curious...
Is there a canonical reason that the Doctor gets progressively younger each time he regenerates?
Ne, no given reason. However, I have my theory is that he's pretending to be the opposite of what he is. As Hartnell he looked really old, but often acted like an anti-social teen. As time progresses he acts more 'mature' while at the same time his appearance looks less abd less so.
At the heart of all beauty lies something inhuman, and these hills, the softness of the sky, the outline of the trees at this very minute lose the illusory meaning with which we clothed them, henceforth more remote than a lost paradise.
-Camus, An Absurd Reasoning
In the 7th doctor era I believe the name of that set of episodes is "Battlefield" the Doctor actually is "Merlin"!
Otherwise I believe its because the choices for each incarnation of the Doctor has steadily grown younger I believe the BBc have got into trouble for removing their older presenters and replacing them with younger versions but thats an argument for another thread!
The doctor has gotten older too, at times. If you look back to the transitions 5 to 6 to 7, each of them was older than the previous.
I think the original intention for Matt Smith's doctor was to pick someone older for the role. I seem to recall an interview from around the time saying that they really wanted someone older, but Smith just knocked it out of the ballpark in his audition so they took him anyway.
Finished Season 1 last night. It was AWESOME, though I didn't quite catch how did the Daleks survive.
Guys, he is expecting ANSWERS in Doctor Who.
Sorry, but once in a while (and later in Moffat's run more often than not) the answer will be "because it makes for a good plot". That's really what drives the story, not logic and explanations.