He seemingly didn't mature at all, they've written the character as if his personal growth stalled at the age he went missing but in reality growing up as the outsider in a foreign land under strict watch of martial arts masters should suggest he would be the most mature out of all the characters, especially considering how apparently successful he was at it, until he gets infected by the real world and learns to loosen up. Plus in Iron Fist he reminisces about his growing up with Davos, sneaking around etc. so while his adolescence might not have been standardly "western" / "New York" he clearly grew up, but the shows fail to reflect that in any aspect of his character.