Paleolithic hunter-gatherers organized in bands that included several families and consisted of 20-30 people, although some were larger. This closeness afforded protection and a survival advantage. When a couple married they usually lived in the band of one of the spouses. Bands included all generations from infants to the elderly. With the advent of civilization and the protection provided, living in nuclear families, or alone, became possible.
Modern life provides vastly greater opportunities for young adults than the Paleolithic era, yet the path to those opportunities is predictable. For the typical boomer it included high school, college or a job, marriage and having children. Moving out of the family home was a mark of adulthood. However, more recently, many young adults are staying home.