technically, no. intellectually--maybe, maybe not. i'm fifteen and i know for a fact that i'm mature than many kids my age, but definitely less than some as well. i feel like i would define a child as someone who lacks experience. thinks that they know everything. has this sense of entitlement, are more focused on themselves rather than others, are more idealistic than practical, etc. maybe a bit transparent, and don't have a personality formed. or sets of views aligned. one that forms their opinions based on what others say as opposed to their own. things like that.
think they have people figured out. or that they know the things one has experienced over the course of their life to make snap judgments about. not willing to take responsibility for their actions nor think that they should. thinking everything is black and white. having high expectations for people. or even expecting people to bend over backwards for you for no reason. taking things for granted. declaring that you're right about everything, and being condescending/intolerant of people that hold different opinions than you (and since there are many adults that act/think exactly like this, i'd probably consider them child-like too)
and after one reaches a point where they can take a step back, and realize they don't know everything that they think they do, and will listen to what people have to say, are questionable, respectful/accepting towards others for the way they choose to go about their life and the decisions that they make(as long as they're not hurting anyone). become more open minded, and gain experience, and an understanding of the way the world works and it's harsh reality, (life isn't fair, you can't always get what you want, etc) then i wouldn't consider them a child. some are forced into it though. the way i thought last year was much different than how i think now, and it changes all the time.
i look at myself when i was 12/13ish and how great things were, but how i was also naive and unprepared for how sh!tty things were soon going to become. so i feel like as one ages, some get slightly more pretentious because they look at people younger than them and assume that they know just as much as they did when they were that same age (which to them was absolutely nothing compared to the things they know now). and then become astonished to find that they really know just as much as they do, despite their age. and i don't think they'd look at them as a child anymore. so age gaps make sense to me in that context, but i can also see how one could easily be taken advantage of as well.