There is a physical maturity that can be measured. What about mental maturity. When is a person "mature"?
I've decided on one approach. That is, that people are often reacting with situations. When people react, they sometimes do what they want to do, and other times what the situation calls for, regardless of their own preferences. My guess is, that when people do the latter, they are mature. And the fully mature person will never let personal preference get in the way of a decision, unless there is more than one solution, and both are equally good.