Of course, he's an adult. You can vote. You can serve in the military. You can marry. You are out of school. You can own a gun. You are definitely an adult at 21. It is the law of the land that you are an adult at 21. I don't know where ridiculous notions come from that you're not an adult. It's that kind of low expectation that allows 21 year olds to act like they're not adults.