So I was reading on the Art of Manliness about men who still felt like children. As much as I am a "fun dad," I don't relate to this problem. I guess I've felt like a man since my early teens.
It's not as if I didn't understand that I would be more mature and knowledgeable tomorrow than I am today. On the contrary - I was very aware of my growth as a man. I still am. I just view that natural, positive change as, well, something that you should expect to happen. It doesn't make you something less than a man today. There is no magical threshold you cross, despite our attempts to ritualize one into being.
The way I look at it, I am a man today. I was a man yesterday. I will be a man tomorrow.
Perhaps what those man-children are feeling is the desire to play. Perhaps they believe that, when they finally feel like men, the desire to play won't be felt. If so, they're being silly. Maybe they view their hobbies and pastimes as something less-than-manly. They may simply not grasp what a man is.
There are plenty of articles that purport to tell you what a real man is. Each of them lists what the author values, and thus present many points on which to differ and discuss. I'll not add more noise to that channel aside from this: being a man is possessing and developing those traits that one thinks a man should exhibit.
If that leaves you flat, then contrast manliness with childishness. A child is self-centered and dependent. The opposite would be empathetic and a provider. Such a definition applies to being a woman, as well, without resorting to stereotypical gender roles that may or may not apply to anyone in particular.
So, did I care about my fellow human beings and provide the world something of value yesterday? Today? Tomorrow? That is definition enough.