Why do you assume that all families are run the same way?
I know a family where it is expected that the oldest daughter will marry and move in her husband and then have children and what not. Its not wrong, it is just not the norm. As long as the family is okay with it, there really isnt a problem. And just because a adult child is living at home doesnt mean that they are freeloading or dont have lives. That person most likely pays all the bills and or morgage and what not.