I get that.
I'm only 25 so I don't have much responsibilities to hide behind and feel adult. Not that i'd like them, I kinda have a fear of them, sometimes.
Although, being a young adult, I especially get this feeling alongside feeling lost.
When your an adult there isn't anything guiding you, so like the child without guidance you're lost. It's then you see life's pointlessness, because the only point of living were always the arbitrary goals others gave.
In that regard responsibilities are also outside goals that guide you.
In a more joyful note, I don't even know why fun should be childish but it's amazing to be at all age.
Those two facts alone are what makes me think of how similar I still am to the child me.
In Marx's own idea the point were class warfare is no more is when our civilization can satisfy any needs of anyone.
It would be the ultimate goal of communism, perfect equity through infinite automation of all resources.
Then they would only be art, philosophy, science and social activities.
Except, as long as there's limited resources, fighting for it is our nature. To the point of having to much if may be.