I never fully moved back in after leaving for college in 2002 when I was 18. I'll be 39 September 1st. My younger brother just turned 36 and still lives with mom and dad in a town of about 8000 population. He is completely ruined at this point. Failed out of school, worked a few dead end jobs and hasn't had a job in years now. Has never paid rent, asks for reimbursement when he buys groceries. He did have about 25K in savings (cash) when I got tired of telling him about investing years ago and I've given up trying to get him to make something out of himself and move out.
At this point, I'm hoping he just stays there so he can care for them as they get older. They are mid to late 60s now and retired.