The simple fact is, no one really "grows up." Time passes, you age, you take on responsibilities either because you want to, you think you must, or to remove the burden of responsibility from others, but you are forever the same you. You merely gain experience and adapt as needed. The goal is always to survive, nothing more, nothing less. If happiness and contentment can be gained along the way, all the better, but if you don't survive you can't get those things later.
Still, it's not as depressing as it sounds, because there's a secret others don't want you to realize - as long as you answer to the powers that be, everything else is irrelevant. Pay your taxes, follow the law, listen to your boss, respect your loved ones. These are things you must do. But everyone else? They only have the power you give them. Deny them a say, cast out their opinions, and you gain a freedom of sorts. They can be unhappy with the child you were, they can even attempt to discourage it, but they cannot truly stop it. And even then, you'll never be alone, because there are too many people in the world. Somewhere, there will be someone who has let their child out too, and together you may play to your hearts' content.