1. Becoming and adult can mean many things. For some it can mean living on your own and for others it mean getting your first job and being independent.
2. It is usually determined by the state you live in. It is not chosen in the law which mean the state decides when your age is considered and adult.
3. This also has many ways. You will have the understanding of what people used to say as when you were a child. You will have more self-control. You can even gain patience. (Honestly this is an opinion.) This question has many answers.
