Depends on the situation.
If you mean somebody lives with their parents, and is essentially an adult child, who has no responsibillities.... then 18 or 19.
But, lots of people have parents who are having problems, and they can really use the help... merging households = 1 mortgage or rent payment with more people to contribute, it also means people who maybe can't drive any more because they are too old are more likely to get a ride somewhere. Also, means your elder mom won't be living alone, it means she'll have somebody around to keep an eye on her or look after her needs.
Thus, if a 20 year old is living at home, and helping their folks with bills, etc, or if a 30 year old moved back in to keep an eye on mom or dad, then that's perfectly ok.
Family should stick together, especially when times get rough.