Who says we have to grow up so fast? It's almost like once education is over, you're an adult and you're expected to act a certain way and speak a certain way. Responsibilities and pressure starts to increase, fun starts to become a thing of the past.
Now I'm not saying that we should all throw away our responsibilities, in fact, that's absolutely not what I'm saying but it's okay to have fun. It's okay to be a kid and enjoy all the wonders of the world. It doesn't matter if you're 12, 22, or even 62. If you want to build sandcastles at the beach then go for it. If want to swing on a tree then that's completely fine. If you want to tip out your old lego bucket and start constructing lego houses then that's okay too. As long as you're not harming anyone then there really isn't a problem.
Whatever your age, it's important to maintain a youthful spirit. There are so many amazing things in this world, and so many small things that can bring a lot of joy and magic. Don't let a number define when you have to suddenly stop having fun.