if a brother, their father was killed 27 years ago so, the youngest would be probably 26 as the mother could've been pregnant with him at his death. rainier is desperately protective over his family so they'd be fairly close. he would also probably know that he was the one that recently killed the king.
as for the protege, age range can be similar. he'd want someone who could possibly become his king's hand when the time comes. he'd train him to fight, to main the sails along with him (he owns a boat called the argonaut's roar and learn how to manage court life. if he were his protege, he'd also probably know that he recently killed the king.