The biggest thing I realized is teens haven't changed that much in the decade since I turned 20. They still love to hang all over their friends (same and opposite gender). They still write on arms and legs. They still stay up way too late (and make noise even with a toddler sleeping in the tent next door). They're still in a hurry everywhere they go. I forget the intensity of my teen years because I've mellowed with age, but watching a fifteen-year-old rush across the fairgrounds to join the pit for a favorite band, it all comes flooding back.
A lot has changed too. I got my first cellphone when I left for college. It made phone calls on a tiny, black and white screen. Almost every teen I saw had a smart phone - and more often than not, spent more concert time staring at the screen than at the stage. These particular teens seemed less cliquish too. In my days, the goths didn't hang out with the emo kids, who didn't hang out with the hardcore kids, who didn't hang out with the punk kids. I grew up in the Christian rock scene so I think I have a fairly good comparison there and I have to say I recognized a lot more unity than when I was a teen. That's encouraging to me.
Obviously fashion changes (hello Katniss braids!), musical taste changes and technology changes. But so much stays the same. Girls still try to keep their appearances up in dusty camp grounds. Boys are still too shy to profess their feelings for their girl friends. Every small hiccup is still the end of the world. Somehow, I find that reassuring.