Maturity, like many things, is much more complicated than a simple one dimensional trait as common knowledge implies. I think there are two main aspects to maturity which I've decided to refer to as inner maturity and outer maturity.

Outer maturity is easy to see, and people's maturity level is usually judged almost entirely on outer maturity. Someone with low outer maturity might enjoy a snowball fight while someone with very high outer maturity would rather stay inside and sip some tea while reading the paper.

Inner maturity is much trickier, someone with low inner maturity might be quick to anger and frustration, focus on themselves, and make poor decisions particularly when under pressure or stress. Someone with high inner maturity can maintain a certain levelheadedness even in stressful situations, can put others before themselves, and keeps future consequences in mind when making decisions.

Inner maturity is almost impossible to judge accurately until you know a person very well, and can often be easily masked by outer maturity, especially if the person desires to appear to have a higher inner maturity then they really do. Inner maturity can be hard to accurately even see in ourselves, let alone in others.

This is problematic, because inner maturity is the one that really matters. We can't see what others are thinking, and so all we see is the everyday actions they take, which are often governed by their outer maturity (especially in non-stressful scenarios). Outer maturity is almost entirely unimportant, as long as it's not too extreme (on either end) it's more just a way in which people are different and less a way in which the quality of one's character can be judged. Inner maturity is the one that's relevant to things like how able this person is to handle life on their own.

Even splitting it in two like this I think is still a vast simplification of the concept of maturity, but it's a much better way to look at it then as a single thing that's just either high or low.