The character is adopted into a Wealthy family. If he is an orphan, it is a true adoption. If he has living parents, he is treated just like a son. (work with your DM to determine details)
Martial training: In adolescence you trained with the weapons of your people and learnt how to use them proficiently (work with your DM to determine details)
Approaching adulthood you experience wanderlust and decide to travel, roll 1d6 to detirmine years spent travelling and think about where your character might have gone. (work with your DM to determine details)