User blog comment:Jonesy95/Possibilities for Dishonored DLC/@comment-58.168.79.244-20130209135422/@comment-27202257-20130209201844

It seems like, yes, it's the monarch's age that matters in the choosing of the Royal Protector. This is complete conjecture as there's no evidence one way or another regarding it, but it's my guess that if someone very young is chosen to be Royal Protector, the position is retained by the previous Royal Protector or a stand-in until the chosen Protector comes of age.

It's also my thought that Corvo was "gifted" as a diplomatic gesture when he was a child, then formally "assigned" to the Empress after she chose him to be her Protector. But then, what happened before Dishonored is a huge playground for such thoughts, as there's just enough background to build theories around and no more.