How old do you have to be to have surgery?

Technically it can be any age with your parents consent. Due to most parents having morals they usually wont give consent, unless its a medical emergency. Because you are an adult at 18, all parental consent is unessecary in these matters...although by the sounds of it you are quite young and my advice is that you exercise and diet in order to lose weight because it is alot easier for younger adolescents to lose weight than it is if you are older.