I was 18 when the Army removed me from home. After my discharge, I was invited to stay at home for a short period, but was charged rent. I was incensed, but paid it. I soon found a reason to leave permanently. Re-instate the draft and the problem of 18 plus kids at home would be solved. For the really messy kids, make sure they are inducted into the Marines. That's a swift cure for many bad behaviors.
I was a full time student until about 6 months before I moved out and except for my last year at school I always had a job and always paid rent.