answer:It’s not exactly a hard fast rule, it’s more like guidelines, and cultural norms. In my family my parents never would have let me live with them for years after high school without some sort of plan or goal to be able to support myself. In my husband’s family children were expected to live at home until they got married. His brother lived at home until his mid 30’s. There is some sense to our expectations as we learn more about how our brains develop. Previously it was thought by the teenage years our brains were pretty much developed and through experience we became smarter about life. Now, science shows us that the frontal lobe of the brain is still growing and creating connections throughout our teen years and into our early 20’s. It explains why teens still aren’t great at predicting consequences, especially long term consequences and risks. It’s also natural for teens to practice their independence, and the safest way to do it is while still living at home, but challenging their parents authority to a point. College years create an extra buffer of time for children to get more up to speed with the demands of adulthood. Not only the academic knowledge, but literally a stall for time before having to be completely responsible for themselves. Other expectations like when to talk and when to walk in early childhood are averages. Things like when to have a first kiss, first date, get married, have children. Again, those are partly biological readiness and cultural norms.