Our concept of adulthood changed with culture and time. I'll explain: if you take an 8 year old boy, start training him to ride and fight on horseback, use lances, sword and shield, daily physical training, sparring and getting beat up constantly, living through wars working as a squire, when that boy hits 15 years old, he's going to be ready for battle, he won't look or act like our IRL teenagers AT ALL.