Does maturity exist? The popular answer (and best one-word answer) is "no", but I'm realizing that may sometimes be too quick of a dismissal.
Maturity could simply be defined as age. That clearly exists: a clock can measure it. But what most uses of the word refer to are desirable traits commonly associated with relatively higher age.
This, of course, begs the question of what traits are desirable. There are too many to list, so I'll just be using the most correct reference I know (myself) and treat them all as givens for the purpose of this post.
Let's assume for now there are certain desirable traits that increase in degree with age. Returning to my most observable test subject, have I gotten more mature as I've gotten older? I would say yes. In a moral sense (how I think, what I believe at the core, what I value...maybe morality deserves a post), I matured until I was around 16 and have maintained that level since. In a practical sense (knowing what to do and when), I've matured more with each year. And it's the practical side of maturity that liberal attitudes say is overvalued and has nothing to do with the far more important moral side. Yes, being more practical is being more moral, but it's not like a 10 year old who can't fix a car or make a business deal wants to be unable to. I wouldn't describe them as "immature" on that basis. Assume, though, that they have a choice to ask for help. If they didn't do that, then I might call them immature. By understanding the fundamentally enriching nature of cooperation, that person could become more mature.
I'm quickly getting off track here and just deleted a portion with another example of lack of knowledge. We all get it: there's a difference between not knowing how to do something and not choosing to do it. One is a matter of morality, the other of practical knowledge. The dichotomy I went on about begs the question of whether or not people tend to get more mature as they age. I don't think they do. They could reach the peak of their maturity at 20 years, 15 years, 60 years, or even 5 years, and then descend. Their practical knowledge will increase at some rate until their brain starts breaking down, but with greater ability to think in terms of pure practicality can come a greater tendency to ignore the question of why. In my case, I just want to have fun. I also have certain goals that take a spiritual context to explain, but could be worded as "being the best person I can be". If that goal didn't make life more fun, though...well, no need to worry about that!
With that, I believe I have a conclusion regarding how to define "maturity". To sum up, yes, maturity exists, but it is synonymous with morality and has no direct relation to age.
By the way, to a certain audience, I had started on a Google Doc about "whether or not I've changed", as well as a plan for a system of "[group name hidden] Editorials", but the former ended up boiling down to what I said here, and the latter is essentially the purpose of my and one other member's site, though I do hope more members follow suit. You know who you are.)