Royal Protector training

It's been something I've been rather curious about, so I just look at Jessamine's wiki. It said that she made Corvo her royal protector at 12 years old. So, if my math is correct, Corvo was 19. So that makes me ask what is the training like to become a royal protector. How intense is the training? how young do you have to be in order to get in? Is it just like an unknown thing, or is there lore that goes into it?