Certainly you're not an adult, because you're just wandering around, still trying to figure out how life works as you go along.
There's no test, there's no light, and there's no tangible event that signifies the transition into adulthood. You don't enter an organization of adulthood. There's not, like, a guy hanging around at your college graduation who comes up to you and says "Now that you're an adult, here's all of our secret knowledge about existence, and how to live as a functioning human outside of the school system."
"Since you've graduated I can legally give you this box, which is just FULL of all of the rules that govern life and existence."
There's no class or test or paperwork to sign. One day you just realize you're a person who pays bills. You're a person who signs up for a club card at your local grocery store because, "Oh, I might as well, I'm there so often." You're a person who gradually is getting less and less familiar with whatever's going on in pop music. You can vote and rent a car and get married and have kids, and it's not weird, it's normal.