I listened to an interview on NPR this morning with violinist Itzhak Perlman. They asked if he knew more about the violin now, as he turns 70, than when he was 20. 

He said "no, but.." and paused.

At this point my brain filled in the answer.

"But I know myself much better!"

Turns out that isn't what he said, but it's still an important idea that I'd like to pass on to my younger friends (I am 60). 

When you're 20 you don't even really see yourself. You and the world are the same thing. That's why young people feel there is such a thing as absolute right and wrong in all cases. The world seems simple. It's all about me! And anything that I don't like obviously is wrong, and anything I do like is equally obviously right.

What happens as you grow older is that this sense of being everything can fade away, and as it does, other people and things become visible. You see that there are lots of different types of people, with different experiences, different ways of viewing the world. You can delight in this, and learn from it, and use it to further define yourself. 

At 60, I often laugh at myself: "Oh that's just something Dave does."

That would have never occurred to me at 20.

On the other hand, not to say there aren't wonderful things about being 20. Everything is so fresh and new, the world and time seem unlimited, and your abilities. Falling in love at any age is a miracle.  And there are rewards that only come from knowledge and experience. 

PS: I'm also a better writer at 60.