User blog comment:Master Sima Yi/Assassinews 12-05-'15 — Assassin's Creed: Syndicate has been revealed!/@comment-109.182.148.62-20150601182947/@comment-13471981-20150601190656

I guess 21 is a nice balance? 18 would technically be an adult, but contemporary audiences might consider it a bit young for a trained killer (I dunno, I'm guessing here). 21 still feels far away enough from mid-twenties to be "young", while also being around the age age at which people are assumed to have come into their own, with their personalities fully developed.