You shouldn't be treated like a little kid. Because you're way more than just being "innocent" or "silly" or whatever, and acting like a child is fun, it at least makes you unique.
Who made you feel like that was a bad thing? To be "silly" and "innocent."
Why would you want to fit in?
What does "fitting in" look like?
How would you ever "be like other people" when everyone is different?
Why would you want to belong?
Why can't you just be you, whoever that is, and have people take it or leave it? If they don't like you, so be it.
And why do you feel like you don't belong?
Who are you trying to be like?