I have been a parent since I was 15. Officially married and moved on our own at 19. Graduated from the university at 22. Struggled hard-core until about 30 when my career changed and finally kicked off. My wife became an at home mom for our now three kids. It was my 40s when I realized, "oh, others see me as the adult in the room." I joke and say, "i have always been in my 30s," but I do feel a change recently. Very much facing forest dweller stage already with my oldest getting married.
What makes an adult? I think accepting responsibility for your (and often someone else's) condition is a big part of it. I did that at 15. I double downed at 30 when I became our sole provider. But it was my 40s when I started to feel like an adult.
I see many "adult" children and many more adults acting like children. The difference seems to be a combination of self-awareness, social awareness, and responsibility taking.