Depends on the person and the culture.
Also on the economic climate.
When your grandparents got married at 18 or 20 in the 1950s, they could afford to move out because things were more affordable and if they had a union job with General Motors or whatever, they had a good amount of security. And I think a number of post-war veterans did well with the GI Bill, or whatever. Someone correct me if I'm wrong, though.
In some countries people live at home until they get married.
Today in America, I think people start to look at you funny once you creep into your upper 20s.
I'm not one to judge others though because who knows what their financial situation is like.
Maybe they are saving money to buy a home.
A lot of people live at home or do the roommate thing and just pocket cash.
I wouldn't blame them. Sometimes people have tens of thousands of dollars to pay back in student loans...or maybe they just like the financial flexibility it offers them. They can travel, eat out, etc...