When do you become an adult?

You become an adult when you turn 18 chronological earth years old. When you turn 18 you go to college and you move out of your house. You learn to take care of yourself (Such as those annoying taxes) and learn financial management. You get the right to vote and are responsible for following the law and paying for fines. When you turn 18 parents learn to respect your decisions and you have the burden of being on your own.

Some people become an adult when you reach a certain level of maturity. Maturity comes with age, so if a 16 year old can act like an adult should act and can take care of all the responsibilities an adult takes care of, then they are an adult. To be an adult you need to act like one. That means that you don't just waste your money on meaningless objects, you budget and plan. Instead of buying video games, you spend money on groceries and taxes and making sure that you can get a house and manage a family. 

My opinion is that to be an adult you need to turn 18 years old. By this point, you have graduated high school, or not, and you know what your plan is for your life in the future. You take a look at the problems, you solve the math. "How should I save for tuition?"  "How much groceries should I buy with a certain amount of money?" Your free-time doesn't happen when you get back to your dorm after doing classes anymore, your free-time is when you get your work done and you have whatever spare time left over. You don't have the time to procrastinate or play games or whatever. You have a budget and a plan, you have a job and you can preform it well.