High school was the best time in my life until something better came along. My senior year of high school, my group of friends were treated like royalty. We were pretty popular and generally well-liked by all the other students, we were in honors/AP classes so the teachers liked us, and we were on sports teams so that covered being part of most things in the school. We could leave class when we wanted, get early dismissal notes from the administration, and got special treatment from the cafeteria workers with a little sweet talk (they made a new type of donut at the snack bar just for us).
When I went off to college, it wasn't the "super crazy non-stop party" that everyone made it out to be. Your college experience is definitely what you make of it, and I decided I wanted it to be about academics. I had always been a little more introverted anyways, and without a large group of friends around throwing myself into the college party scene was a little intimidating. Whenever I'd see my friends when we were all home for holidays we'd chat and reminisce about how good we had it in high school. I remember thinking how sad it was that my life had already peaked; I didn't want to be the person that sat around a bar at 40 going on about his achievements from 25 years ago. I eventually made it through college and then moved on to the next best time of my life.
After college, I got a job in a great city for young adults, and had a couple close friends in the area. You never quite have the same freedom as when you get your first job out of college, you're making decent money, and have no ties or responsibilities outside of your work. One of my friends was the head of a frat while in school so he knew a lot of people, which was great for me since I'm terrible at meeting people on my own. We would meet up for happy hours on weeknights, go out to clubs on the weekends and wake up around noon in time to hit the river for a few hours to go out again. We would take random trips all around the country just because we had the money and the time. Some of the best friends I have I made during that time, and we still meet up every now and then even though we're spread around the country now.
I'm currently in more of a "slump" period now, not that my life isn't great, but it's not quite the non-stop party it was a couple years ago. Once I progress further in my career I can see myself traveling the world and getting back to that level of freedom I had before. I guess what I'm trying to get across is, what you consider the "best years of your life" change as you get older and your priorities change. I'd look at it more as the "best years of your life so far" because you never know what could be ahead.
Source: 26 year old who has experienced multiple "best years of your life" in the short time I've been around.