Brass tacks: the only notable difference between a 20 year old and a 15 year old is the year of birth on their ID. Emotionally, there's not a difference worth remarking upon.
From my perspective I don't think most people mature until their late twenties. I say that from my own experience of having a deeper understanding of context in things, plus the conversations I had with people around my age and older are also going through the same wisdom transition as me. I also think there are some people who don't mature mentally throughout their entire life.