Being grown up, imo, is being self-sufficient. You pay taxes, your bills, can drive yourself to the airport, etc. Of course that doesn't mean you can't have help from a SO or family/friends, but if you needed to say, make a doctors appointment or go to the DMV you could do it all yourself.
That's it. Being an adult has nothing to do with a state of mind, or popping out kids, or whatever else. Every single year I grow older my life gets better, because I actively put time and energy into making it better. I would never ever EVER want to go back to my teenage/childhood years of having no autonomy. If I want to hop in a plane and take a trip to Washington DC to browse the sights alone, I can, and I have. (Note, not everyone is lucky enough to have a stable income, and I hope for those who are reading in that position that you find a good job and things get better for you.)
I also have no plan of getting married or having kids of my own, things that are consistently related to being an "adult" which I find tiring since life shouldn't be a step-by-step guide of grade school>middle school>high school>college>job>married>kids>retire>die. The thing that keeps life exciting is your ability to make your own choices and do things at whatever pace you want. A serious consideration I have in my life is once I hit my 50s I want to maybe start taking in foster kids in their teenage years, and that is not a conventional thing to do but it's something I can do!
Being an "adult" doesn't mean you have to start being disenchanted with life and settle into a dead-end marriage with 2.5 kids. You can still go to anime cons when you're 60, cosplaying as Kaneki Ken with your Pokemon Ita bag dangling from your side. Don't fall into a trap that being an adult means you have to change into someone else, live your life however you want and never give up the things you love.