Well it is more his character, he will still be the same when he will be 40 yo, it is not something you grow out when get older unless get some serious help, want to change and even then it may not work. MC is not a kid as such, but a psychopath that completely acts out of his needs and wants and can't look beyond it. Which is childish and narcistic as with most psychopaths, but does not change anymore on its own when get older at age of MC.
That is what psychopath means in a way, not having grown and developed the appropriate skills in the development age you should have or slightly later to be a nice productive member of society conform the rules. If the lack of those skills mainly hurt yourself it is a mental health problem, if they do others you are a psychopath.