I’ll go against the grain here and say that the OP is probably dead on. Whatever it is you are doing that you think you are now too mature to do? Yeah, it’s time to quit.
You are no longer cute, precocious, or a wunderkind. Stupid mistakes are now plain old stupid, and hurting people is no longer assumed to be accidental. You must now view your parents as fellow humans and be polite towards them and grateful for any support they provide. You can’t sleep with that sweet young(er) thing, unless you want to live under a bridge or behind bars.
But adult fun can be more fun than young fun, provided you seek out actual fun instead of settling for the chemical (or video) substitutions. Both kinds are more and more available, so be the guy with a frisbee in his backpack who knows the rules to Charades and Botticelli.
Another suggestion: Some of the best fun and friends I have found in adulthood have been in the course of performing charity work. Find a Habitat for Humanity build, or a USO care-package party to contribute to. Whatever type of worldly mess bothers you most, find a group of folks trying to help clean it up, and go put your hand in. THAT is where happiness can be found.
The one birthday to actually be anxious about is your 37th. Because at 37 you have been (at least legally) an adult longer than you were a kid. Nobody truly appreciates their youth as much as those many years removed from it.
I think your 100th birthday is the one to be anxious about. The closer you get to it, the more you really want to make it, but the less chance of getting there.
I’m 45. I always believed the future would arrive, so I always made sure I was fit and making some money. I’m having a great time, and hope to chart a course to philanthropy by the time I am 60 or so. I do wish I had known about yoga when I was 20…
There’s something to be said for 20 being the biggest milestone for you to date. But it is the beginning of your adult life, the life that is going to be, yanno, “your life” by the time you’re done. You can do anything! “Juvenalia” is a footnote for most people, just sayin’.
“Humility is the gateway to the virtues” is a good thing to know, so I’ll tell you that. If you don’t drink, I’d keep that up. Not much besides cigarettes and opiates are worse for your health, and your body will withstand all kinds of other abuse better if you just don’t drink. I’m not recommending doing a bunch of drugs, you can honestly and for true screw yourself up that way, but I also think just about any other drug in moderation is better, health wise, than alcohol.
Be curious. Don’t be a slave, but do work hard for your own interest. Develop your mind. Learn Euclidean geometry and formal logic if you haven’t already, and also pursue some kind of art form, even if just as a hobby. You really do need to engage in some strict formal discipline, but understand that this will ideally be a kind of skeleton for all the squishy life that will accrue around it in time.
It’s possible that we don’t even have free will, in which case all you can do is try to enjoy the show. Good luck and I hope it goes well for you.