I also think, there's a difference between feeling like a child and feeling immature or inferior!
I too associate with child (not with a personal childhood) positive things: Lots of energy, lightheartedness, curiosity, impartiality, no feeling for time, feeling the whole world revolves around you etc.
In a study a psychologist asked people how they define being an adult. He expected things like "build a house, have a family/son, plant a tree..." But it basically all boiled down to 3 things:
Taking over responsibility for yourself!
Accepting and dealing with the consequences of your actions and decisions without anybody else protecting you from it! Esp not your parents. And without blaming anybody else for it!
Making your own decisions!
What job do I want to have? Where do I want to live? Do I want a relationship or do I want to be independent?
Basically the search for your own individual identity! Younger people are more impatient and often there is also lots of pressure from family and friends. But around 30 most people will have found out who they are, what they want for themselves.
The 3rd hurdle of becoming an adult: Paying for your own stuff!
So, you can still be an adult, even if you are still sleeping in your sponge bob bedclothes, watching cartoons or singing rock songs into your hairbrush in front of a mirror!