I don’t know, maybe I’m not fully an “adult” yet, but for me it was letting go of my anger. Stupid teenage angst, I was an angry miserable fuck. Over time I let it all go and I feel like since that time I’ve matured immensely and become a better person; I still have a long way to go, but it’s a vast improvement over what was. I think having a degree, job, supporting yourself all count for something, but not as much as maturity as far as adulthood goes imo.
Oh the number of people I know in their 50’s that still act like children….