18 is an age where a person needs to be transitioning into adulthood and the behaviours and responsibilities that go along with that. That means less time on the leisure activities associated with the common teenager (like spending all day playing computer games and watching YouTube), and more time on choosing and investing time in a career or education pathway.
And if that child is not getting the message that the time for that is now, then it's up to the parents to enforce that boundary.
And if that child is not getting the message that the time for that is now, then it's up to the parents to enforce that boundary.