When I was in my teens being considered a mature person, or a grown-up, was a defining characteristic for me to such an extent that I would weigh almost all my actions based on whether it made me look like an adult or not. I would only do things that did. Therefore I used to (read: pretended to) read the paper every day, drank coffee from an early age, was interested in politics, started collecting a certain set of tableware when I was twelve years old or so, was always home early, paid my own bills (namely cellphone and magazines), and never, ever asked for money besides the modest monthly allowance I was given. I didn't drink alcohol until I was eighteen and even then I only had one beer occasionally for cultural conventions. I took great pride in all this, even felt superior to other teens acting their age.
Little did I know that the desire to be and feel like an adult was not my own, but the result of environmental conditioning, aka I wanted to make my parents proud by doing things the society considered to be the attributes of a proper citizen.
How silly of me, to think that the definition of an adult included having matching tableware. (I have since given the tableware away and gotten a mixed patch consisting of pieces more to my liking, part of which see picture above.) Or reading the paper? Haha! I wrote a whole article about this. I still enjoy coffee because it tastes good but I'm sure it didn't when I started.
Some say the definition of a grown-up is having no posters on your walls. Some agree with Jordan Peterson that being a parent is the precondition for grown-up-hood. Some think it's about financial independence.
Not having any children I can so far only agree with the last suggestion - that being a grown-up means you enjoy financial autonomy by making your own living, one way or another, and taking care of your finances and other everyday issues (like making doctor's appointments and grocery shopping) completely by yourself. But there is one more definition, one that I think deserves as much recognition as this one.
Being a grown-up means that you're able to make your own mind, that no one else dictates what you can and cannot think, say, or do.
This is where some people get it wrong. They think it's "selfish" to only rely on one's own opinions. But it isn't. This definition does not say you should take advantage over other people, nor does it say you shouldn't listen to what other people have to say. It only says you make the final decisions about your own life. Your parents or other authoritative figures do not know better than you what you should do with it. (Don't agree blindly even with this article and let it dictate the definition for you. Think it over. Decide for yourself.)
This is also what I got wrong in my teens. I let society influence what I considered right and wrong behaviour. And I'm sure it still does, to some extent at least. In time I'll find out how and re-evaluate its influence and correctness in my life.
Place nothing above the verdict of your own mind.
P.S. I'm still quite proud I paid my own bills from an early age, though. I learned a lot about handling money.