The person I'm thinking of is Kiran, who's fifteen and lives next door to my family in Pokhara. I've known him since he was a baby, because his mother and mine are close friends.
What shows his maturity most is how he's handled a difficult situation at home. His father works in Qatar and only comes home once every two years, and a couple of years ago his mother had an operation that left her unable to work for several months. Kiran basically became the man of the house at thirteen. He got up early to take his little sister to school, did the shopping, and somehow kept his marks up. He even worked out a weekly budget with his mother so that the money his father sent would last.
But it's not just responsibility. He's also very calm in conversations. If adults disagree at a family gathering, he listens and asks sensible questions rather than taking sides. When I was helping him with English last month, he told me he wanted to become an engineer, but he also explained, quite seriously, that he'd consider a diploma first if university turned out to be too expensive. Most fifteen-year-olds I know aren't thinking that far ahead.
I think he's mature partly because life forced him to be. When you have real responsibilities early, you learn quickly that your choices affect other people. But it's also his character. He's naturally thoughtful, and he watches how adults behave and learns from them. Sometimes I worry that he's missed out on being a carefree teenager, so I make sure to drag him to the lake for a game of football whenever I can.