This is actually a more difficult question to answer than is immediately apparent. I've never really had my own concept of maturity; it's always been something that has come from external sources. When I was younger it was based on my parent's ideology because, of course, like most children I wanted to be treated as an adult by my parents when I was a kid, which meant I needed to fit their definition for what made someone mature. Obviously I failed spectacularly in this regard...and still do, sometimes.
It always seems to be based upon this bizarre idea that the majority opinion makes for a fact, on the concept that conformity provides us with an absolute that we MUST obey, or heaven help us. That is how society functions, but I don't see why I'd need to apply it to my own personal perspective. I don't see why I need to compare myself based on other people's expectations, when said expectations don't matter to me and are impossible to truly gauge because putting them into words is not something typically done and is so difficult to do. These things just aren't done; you're supposed to KNOW how to act in certain situations. I suppose part of maturity is responding accordingly when you know how to...which is conforming, which you get better at doing the older you get.
I enjoy a lot of things that are aimed at people younger than myself, which would classify me as immature to some - stuff that is marketed at children; Pokemon, Pretty Cure, etc. But my behaviour is completely different from those people. At the same time, behaviour is often the product of experience, and I've been here longer, so of course my behaviour is going to be different. I know things the average ten year old doesn't; that's to be expected. The idea that maturity can be judged based on your hobbies or interests is ridiculous. There is a clear line between mature behaviour and everything else that I don't think a lot of people bother to distinguish these days - there is the assumption that if you like certain things, you're immature. A lot of people seem to have that perspective, which is what makes it such a difficult thing to judge - since maturity is based a lot on majority perspective, is this aspect of it a valid addition, or a petty judgement you can safely ignore when evaluating yourself? I wonder sometimes.
The psychological definition for maturity is "the ability to respond to your surroundings in an appropriate manner", which is difficult to measure objectively, because everyone has different experiences, as well as beliefs and opinions of what is "appropriate" for any given situation. In a general sense, I would say I'm perfectly mature for an adult, because I know generally how to respond in an acceptable fashion in most social situations. At least, I can get by without causing offense or a spectacle, and will do so unless I feel the need to express myself for whatever reason. That sounds arrogant, but it's what you would expect of someone around my age; the average person could say the same. We wouldn't function as a society otherwise if we didn't have that knowledge, and we're not born with it - we learn it from others as we grow older.
Beyond that, how mature I am depends on your perspective. I stopped caring about it a long time ago; I am who I am. Take it or leave it or, as is often the case, both. If I'm immature because of what I enjoy, how I act in certain situations, or because of anything else, then I'm sure I'll survive somehow. Maturity implies a perfection that just isn't present in people to me; no matter what I do, at least some aspect of me is going to be immature to someone, somewhere. No sense worrying about it; we get by as best we can.
tl;dr "mature" is more a word that other people might use to describe me (or not) than one I would apply to myself.
