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.
Why do people have kids then??? LOL