Growing up means maturing. It means becoming independent. It means developing into your own person and standing on your own two feet.

Few people really do this. Because few people ever get around to extracting all the beliefs that were placed in their heads by other people.

Most people have similar beliefs to their parents about life, society, politics and religion. If it's not their parents it's other influential figures in their lives like friends, teachers, or the media pundits they follow. 

More significantly, most people hold a lot of beliefs about themselves and their place in the world that were put in their heads by people they care about. The average person goes from cradle to grave without ever seriously examining their beliefs about who they are, what they're like, or any of the other ideas about themselves that get inserted into their minds by the verbal and nonverbal communications of the people around them.

For most people, a huge amount of their personality is pretty much locked into place in early childhood by the life experiences that they have with other people during that time. They're still essentially who they were when they were kids, but we call them grown ups just because they're a certain age and can have kids of their own.