3. The Capaldi Regeneration Is A Sign That The Doctor Has Forgiven Himself
This was one that I thought after the 50th anniversary too and I'm pleased to see it pop up in a couple of places online. Following the Time War, the Doctor only regenerated into younger versions of himself. This is based on the quote from the War Doctor saying "Do you have to talk like children, what makes you ashamed of being a grown up?" Perhaps the thing that makes them ashamed of being an older version of the Doctor is the perceived actions of the War Doctor himself? This regression into younger and younger forms could be the immature response to his past, more wizened incarnation, somehow subconsciously affecting the regenerations to become younger and younger. Of course with the change to the 50th Anniversary where Gallifrey Falls No More, he no longer sees the big bad version of himself being one of the older versions. So he is no longer afraid of being old thus resulting in the older Capaldi regeneration (remeber, he's the oldest actor to take the role since William Hartnell) because he no longer feels guilty from his actions on the last day of the Time War. I'm not sure if it was deliberate by Moffat and his writing den, but it certainly works for me.