Getting into Doctor Who.

[Weezer]

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.