assassins creed story confusion

spoilers for Assassins Creed: Revalations [ok, I have played all of the console assassins creed games, and at no point aside from this one did they ever mention, at least to my knowledge, that Desmond was born into the assassins brotherhood and raised into it. Did I miss this bombshell or is it a retcon?

tl/dr: since when was Desmond an Assassin from birth?](/spoiler)