Can you please tell me what this means. I don't understand it thank you.

I had never quite understood the dividing line between not growing up and being grown up. It seemed as though there was some invisible doorway a person walked through and when he came out on the other side he was suppose to be all that he had been before, except more.

2 answers

its talking about not quite understanding maturity in which a young person grows up to become an adult. for some its like watking through a doorway and for others it seems they can't even get the door open.
The person is confused as to what being "grown up" really means.