Not everyone reaches milestones at the same time and not everyone lives the same life. I find when people talk about youth being the best years of their life it's usually like some kind of story the media is telling people using movies that used 30-year-olds to play teenagers. Or society tells us we miss every opportunity we don't take because capitalism is very opportunistic. Sometimes so much is out of our control that even if we wanted to do something we wouldn't have been able to do it anyway. It's a terrible thing to think about.
There's meaning in causality. We all get old. Even beauty and youth are finite and the human body is meant to decay and entropy affects everything. In the long run, everything naturally decays, and disorder always increases.
You can try to make up for anything you feel you missed out on in your teenage years with other people who are in their 20s who feel the same as you. It won't be perfect and it won't be the same but nothing is perfect and really nothing is ever truly the same between two experiences even when they are very similar.
I've been where you are. I'm much older now though and I never realized what "old" people meant when they said "I feel a lot younger than I look/feel." until I got a lot older. Your body ages but the way you feel is more constant over time. You can still be young of heart and sometimes getting older can free you to experience things you couldn't when you were much younger because being an adult forces you to do things you might not have had to do when you were a teenager. There's more freedom and yet also a lot more responsibilities. It can be scary. The world is a strange and interesting place but it's also boring and average. I guess it depends on how you appraise it.
Everything is constantly slipping into the future. That is one thing none of us can escape as biological organisms. I hope you find some relief somewhere in this world. Just know a lot of people missed out on a lot of things. And we can be our harshest critics. But if anyone can believe that they can miss out on anything then I think they can believe that they can make up for things too. Life is all about self-discovery and telling your own story. Sometimes just letting yourself be wild and free is the easiest way to be your most authentic self. But it can take years and decades to discover who you really are. Taking chances opens you up and makes you vulnerable and the world has sharp teeth and a gaping maw but the risks can be rewarding. Most people you meet will only ever be strangers or acquaintances. I wish the world was more like what the media and our imaginations tell us it is. But even the best stories we tell to each other are often exaggerated because it's very rare for something to be so magnificent. Even our own minds make the best of our memories so much better.
TL;DR: Getting old sucks but you can feel young a lot longer than you can literally be young. I hope you find some relief in this world and other people who have had similar experiences to you and are around your age to help ease your burdens and soothe your soul. Sometimes finding those people makes up for a lot of it because together you can build even better memories. Good luck to you.
And I hope you have a wonderful 20th birthday.
