When Does the Mind Close?

This Christmas we were with relatives and people of every age. And being with them made me wonder again something I have wondered since I was about 16: When do people's minds close?

Is there an age until which people are open, and then their mind closes? Is it caused by an event? Boredom? Socialization?

As a child I remember vividly how different my Godmother seemed. She was old, yes, and wise, but so open. She was curious, about me and what I thought. About new books. About the news. About new ideas. She knew so much, but she was as inquisitive as if she knew nothing. I knew so few adults like that I found her remarkable. Something in her remained soft, flexible, young. Not naive. Very wise. But open.

Other people seem to close up at age 20. It is as if all new inputs of information after that age will fit into pre-existing structures. New ideas, new experiences simply cannot penetrate. They have ossified. It is over. Of course they keep living. But their minds are closed. It is like a grate that shuts over their minds and hearts. Tiny slivers of things will go in through the grate--but nothing in its entirety--not the wonder, the ecstasy of something totally new. It will be made very very small so it can fit through the tiny slats. And then it won't feel so remarkable anymore.

What do you think? Do you think people's minds close? Why? What causes it? Is it just the passage of time? Or something traumatic?