"Maturity is a psychological term used to indicate how a person responds to the circumstances or environment in an appropriate and adaptive manner. This response is generally learned rather than instinctive, and is not determined by one's age. Maturity also encompasses being aware of the correct time and place to behave and knowing when to act appropriately, according to the situation."
Yes, I am willing to apply that to myself. Or maybe I am just introverted and timid to the point that acting out becomes an indesirable option, because I fear that it might inspire the antipathy of my superiors? Guess we'll never know.
I think that maturity is a very tricky thing to measure- behavior is not entirely indicative of maturity, because some people make a mature decision in accepting the fact that they need to express and embrace childlike behavior once in a while, to retain their sanity. For example, I occasionally find myself making horrific, uncontexualised coitus-related jokes with some very close peers. Why? Do I really find them so amusing? No, but I need to vent.
But then, some are just immature. No easy way to tell.
EDIT: I do pride myself on my "Uhm, guys? Maybe we shouldn't do this incredibly stupid thing you have planned, whaddya say?" sense, though I'm usually ignored. I find it helps to stand off to the side as they set the vodka bottle on fire, or whatever it is they want to do.