Haha, I can tell already I'm going to keep thinking of that Bagheerer man cub picture throughout the evening and laughing.
IMO a requirement would be to have developed some sort of moral compass. Also, have a capacity to be conciliatory and reasonable.
IMHO children being immature, and adults being mature is a stereotype I was hardly childish my entire childhood. I don't know if I was mature or acted like one. But I was certainly considered to be mature. The first time I was childish was when I was doing my Masters. Why? I found my true friends by then. I was also very childish with my first requited love. Why? Because we were very intimate.