Twenty was nothing. I was in college, not old enough to drink, and had voted when I was 18 (happened there was a Presidential election that year). So, very little changed for me.
Thirty, on the other hand, was hard. I had just started grad school, and I was literally the second-oldest of the cohort of 67 with which I started. There I was with a bunch of 20-somethings fresh out of undergrad or with only a couple years in the working world, and I had been in the corporate world for eight years by then. I went from being the youngest on the team (at work) to feeling like I was the world-worn wise elder statesman in the blink of an eye. Then I turned 30 pretty much right then at the same time. And suddenly I found myself feeling old.
My thoughts? Roll with it, whatever it feels like. Chances are good that if you're feeling like there's something to turning X, there's more at play than just the number. What you're up with your life and how you're conducting yourself are way more important than a number that society uses to preconceive how it feels you should act.