This is a question that's going to get answers depending on the culture of the people responding to it, no?  If I'm not mistaken, it's far more acceptable for young adults to live with their parents rent-free in Asian countries than it is in the Americas (for example).

Since I can't answer for any geographic region than my own, I'd say it's more than fair for parents to expect that their child/children pay some sort of rent if they're over the age of, say eighteen.  But even then it would depend on whether the "child" has the ability to pay rent and how generous the parents are.  Times have changed, and I'm not of the current generation; what I hear from other people is that their kids are living with them for periods of time far longer than what would have been acceptable about 30 years ago.

I've got no issues with that, since I know how brutal the job market is and how much a college education has been devalued over the past few decades.  But even if a young adult is staying in their parents' home and has an income that's non-zero, they should be chipping in for expenses if only out of respect to the parents who raised them.