I don't know if I started feeling that way when I turned 20, but I definitely know what you're talking about.
My first indication that I was growing up was when I realized that the things that my mom had told me growing up actually made sense. All this time I thought she was a heartless dictator (ex. I forbid you to date so and so--I have a feeling he's bad news), but I understood why she did the things she did. It was then that I knew I was growing up.
And now I see my next generation of cousins going through the same things. My high school age cousins often relied on me to defend them in their parent/child battles (I had a wonderful reputation growing up

), but they don't anymore because they think I've lost my cool.
It was a freaky feeling at first, but in the long run I actually felt good about it...and I'm sure you will too.
------------------
A single seed...becomes a Rose of incredible beauty.
A single grain of sand...becomes a Pearl of great worth.
A single great idea...becomes a Century of success.
Go Alpha Gam!