Biblically, one leaves the family home when one gets married. In some cultures and in biblical times, this was often before the age of 18.
In the U.S., the legal age of adulthood is 18. By then, one is either done with high school or finishing up their senior year. Many that continue their education in college do so at a college away from their family home, maybe even out of the state.
Many that go to college also do so locally. Some may opt to move out of the family home to declare their independence. They may even see it as a rite of passage. Some stay with their families while they may work, go to school, or do a bit of both.
I see nothing wrong with any of this. It's really about what kind of relationship one has with the family. If they are staying at home, are they contributing to the family household by paying rent, doing chores, etc.? Some have the agreement that they can stay with mom and dad as long as they are a full time student and are earning satisfactory grades. To me this is a bit controlling, but then again, many who are legally an adult are really not ready for all the responsibilities of adulthood, and if it works, so be it.
This was probably more info than you wanted.