Back in the day, graduating  from High School meant that we could finally move out!!! Everybody got room mates. That's just what the expectation was. Staying home with the parents was an indicator of failure. Sorry to admit this, but to me it still is. As for the taking care of one's parents cover - most parents don't need care until they're - what? 80? 
The people I know who are over 18 and still parent-dependent don't seem inclined to be headed anywhere. I think it's a disservice to kids to continue to treat them as dependents - or, at twenty something, freeloaders.
Avoiding marriage doesn't wash, either. Living in sin, shackng ip, still viable
options.