As Bluelemond and Smooth Player said, you have to observe what he does that makes people think he is "mature."
I do have a view on this however. I feel that the true test of maturity rises when a conflict is presented. In this situation, you can get the real character of a person and see what type of person they really are.
Maybe this guy does really well when conflicts/challenges are presented? But again, I really don't know. Just see what he does.