I am not aware of a formal test or measure, but I think the concept of "maturity" can be a bit subjective. For example, my first instinct would be to say my son is more "mature" for his age than my daughter was at the same age. DS is more serious, compliant, and self-controlled. Then again, DD, at the same age was MUCH more self-confident and expressive. She would walk up to ANYONE and strike up a conversation (DS might become mute, as he is shy). She is also NOT easily intimidated, and would walk into a new situation and enjoy the experience. So one could argue, that in some respects, SHE was more mature at the same age.
I suppose my image of an extremely mature kindergartener would be a child who is both self-confident like my DD was (and is), and more self-controlled like my DS.
