Is there a better way to measure or test maturity (mental or physical) than age?

As a minor, this question comes up every now and again. I'm sure many of you have been restricted by your age, but thought you were more than mature enough to do whatever it was. I think age has been used far too long as a measurement of maturity. Sometimes the age limit is too low and for others too high. Since we all mature at different rates, why would time be used as the measure of maturity? I sort of aimed this question toward neuroscientists (or anyone else studying the brain) to see if there was already a device that measured maturity or if it wasn't even possible.

Feel free to post relevant examples and of course ideas of how we can either test or measure a person's maturity (mental or physical) better.