You can be under your parents' health insurance until age 26 and you can't rent a car until around 25. Some people talk about the brain not being fully developed until around 25.

But people have been getting married and having children in the early to mid teens for millennia, this thing where "you're a child until 18" is pretty new. Our society does not support young people being able to start families and afford anything in life at a young age anymore, so it's kind of forced a couple generations of people to delay marriage and starting families and general independence. 30 feels like the new 18, and something like half of people between 20 and 30 live with their parents.

Again, that's newly taboo- people of all ages lived together in households for millennia, and our modern Western society has split families and moved us away from our homelands and our roots at unprecedented rates. In most of the world, it's not a strange thing for a 40 year old man to live with his parents or a 30 year old woman. It is a strange thing for them to be single still at that age in many societies.