I'm curious how A) children do it, B) apprentices do it, and C) how this applies when you're improving kids in a campaign. Are there any hard limits?
Speaking of which, is the 45 xp + native language a hard rule for development up to age 5?
Not if you take "Feral Upbringing" flaw, in which case you have 120 xp to spend on a number of suggested abilities but NO native language to start with (save for animal noises perhaps).
Which, not so incidentally, amounts to the same amount of experience. 
That's incidental though to the example 
Depends on the Virtue, really.
A goodly number of virtues are by dint of birth. You're Wealthy or you're not. You're of Faerie Blood or you're not. Unless you have The Gift, you're probably born with Second Sight or Shapeshifting - or there has to be a story reason why you get it later; it can't be automatic.
Other status Virtues take the place of roleplaying a child, like Educated, so one gains them merely by roleplaying them.