[bluefirebrand]

Surely there must be a line you can take between letting them use their free time how they want, and enabling them to never need to do anything else.

What my parents did with me when I was 18 is: I could love at home rent free if I was enrolled in classes at university, or I could start paying rent.

Frankly, I don't see much of a problem with someone staying at home into adulthood, many cultures do that already. However the caveat is usually they have to contribute to the household, either with rent or housework or whatever. I don't see that as a problem.