Correct, but 'legally an adult' only means that your parents are no longer in charge and can be blamed or sued for your actions. You have to pay taxes and you can go to work full time and you are allowed to vote, married and more...
But being an adult is something different.
Or do you think 18 year old understand the world or are able to take wise decisions?
Remember, your brain hasn't fully matured till around 28.