Yes, this is true!
One of the reasons why this is true is that the person starts to be independent then, so they no longer describe themselves with respect to their parents.
Another reason is that they by this time have developed personal characteristics upon which they can base their identity.
The main reason would be the environment because of the fact their population is so big, they need enough room for everyone. There's also the connection with having available food source within the environment when the people plan to settle in. There's also the idea of desirable climate which helps them with food sources and allows them to flourish instead of dying off.
