One thing that just hit me and I kind of never understand

If I train an heir until he is like 14-15 years old and then I decide to king him.
Does he like age down or remain the age that he was or what?

I've seen things where people train their heir until they are basically near death and then go and king them. What happens then? Do they remain the age that they were or..???

this is probably a very newbie question and im sorry in advance for that