Kids should have a life plan. I just don't understand the 'get them out when they're 18' thing though. Maybe it really is a cultural thing. My family at home has three generations living in the same big house. Grandmother is upstairs and son is downstairs with his family.
My Father could live in my house. In fact, I have offered several times. But I could not live in my Father's house, and he could not live with it either.
Could be that's just the way it is with fathers and sons in my family. Nineteen-year-old men are "extra men". Nothing is quite so disruptive as "extra men".
The reason to get them out at 18 is because staying at home interferes with several key components of the plan. They cannot be on their own until they learn to be on their own. They cannot learn to be on their own until they are on their own.
This is an inherently messy business. It is better that it occurs when they are 18 and young and strong. Then if there is a blow-up, it's not to late too bring them back and try again later.