Yep at one point I realized, hey, theres no rules except those I put on myself, and limits imposed by my environment.
If I want to go out and play in the mud I can. If I want to stay up and watch movies I can. If I wanna go for a drive to nowhere after work I can. If I want to read more than 1 book at once, I can!
Its actually really great when i think about it. So many others have self imposed limits ("thats kid stuff, I'm too old, its embarrassing" etc) when they could do so much more.
To OP:
20s aren't that great because usually, your friends are dumb, or you're dumb, and probably dont have much money. And if youre in school you have that to worry about all the time.
Also, we are all children in adult bodies. People act grown up but its all for show. No one knows what they're doing and are just as scared as you.
Just keep in shape and eat somewhat healthy in your 20s at least. Even 15 or 30 minutes of workouts a few times a week will save you a lot of pain! This said from someone who "made fun" of gyms and health nuts my whole life. Still do. Or find an activity to get you moving, lije biking or climbing.