The reason normies rave about 20s is because for many people it's exciting, and the opportunities it presents are the best in life.
I went to Uni, got a job in the field I thought I wanted then, moved out from home, dated many girls, got engaged, played loads of sport, quit my job, tried to become an online millionaire, broke up, got real spiritual, travelled extensively, at 27/8 Facebook came alive and I found out what everyone I knew was up to and started adjusting to the sharing life online space. It was exciting.
I should preface that by saying I did well in school, so had essentially all options available to me. My family wasn't wealthy but here that's not a huge deal as everyone can get loans for Uni that don't need to be paid off until they earn a certain amount. I was never good looking but I was fit, healthy, had great prospects and had a completely unjustified level of self confidence that made dating possible despite that. Most importantly I was physically and mentally healthy, which I assume are the biggest blocks to others seeing their 20s the same way.
Sometime during my early 30s, I had lost interest in relationships, my options had shrunk, I wasn't fit or healthy anymore, had no real money or prospects, everything had slowed right down. By mid 30s, I was a complete hermit with no external contact which has continued into my 40s, and likely will until I die. Which isn't a bad thing by any means, just different - in my 20s I liked the excitement, in my 40s I like the solitude, which is just as well, because the excitement of the 20s is gone for good.
I think most people that say 20s is the best time of your life view it like I did. New found freedom, access to the best dating pool in life, young love, best health in life, and a buffet of career and hobby options to choose from. For them, it is the best time in their life and it doesn't last long enough. But not everyone is in the same boat - physical/mental/family/financial/trauma/other issues can cause a huge shift in how that whole field is viewed and experienced so it's not the same for all.