From Ewen McGregor:
"In your 20s, you spend a lot of time being self-conscious about what other people think of you. Then you hit your mid-30s and start to realize they weren't thinking about you that much."
You really don't have to wait until you are in your mid-30s to realize that. It is true when you are a teenager as well (especially in your teen years because everybody is too worried about what people think of them to concentrate on others).