It's a harsh but necessary message to teach our grown children (mostly the sons): Childhood is over at 18. It was great not having much responsibility, being able to play all day, not having to worry about work, bills, taxes, college, resumes, taking care of a house -- but welcome to the real world.
Teaching your son this lesson would be doing him a great service.
For better or worse, childhood ends at 18. Even if a child stays at home, he is still not a child and God does not want him to "play" anymore. He has other duties of state to attend to. He needs to be studying, working, apprenticing, working at a degree, or something along those lines. He should network and socialize with those in his field of interest. And for heaven's sake, help your son find out what he wants to be when he grows up! Parents need to assist their children a bit with choosing a career. Parents shouldn't be so hands-off in this category. The world is a lot more complicated than it was in the past.
But, long story short, he needs to be a man, not a boy -- as fun as being a boy was.