Can someone please tell me what age you have to be?

The definition of an adult is fully developed and mature. What throws most people into the "not an adult" category is the mature part. You can find some very mature and fully developed people at a very young age but not very many. Some people think it's the magical age of 18 but this is just a legal definition. Twenty-one is the legal drinking age because the law recognizes that at this age, you're mature enough to drink sensibly. So this works for me as well as I believe most individuals are matured enough to be considered an adult at twenty-one.