I think it's got something to do with being willing to take responsibility...and maybe learning to admit that sometimes, you're wrong. I dunno, really, I'd have to think about it more.
I agree with taking responsibility for ones self and making decisions that are for the good of everyone involved in the person's life. Unselfishness, would be a sign of maturity. Taking care of your health and treating others with respect. Doing what needs to be done. Knowing that smoking, drinking and drugs aren't a sign of maturity.
I think that maturity comes from being able to be at peace and have an understanding with the world around us, not necessarily all the answers and explanations, but being at peace with it and ourselves. For some this takes many years, and like I said before, some never reach it.
It depends on how old I am. For example, when I was fifteen, I thought maturity came at age fifteen. Now I'm eighteen, and looking back, and can see how immature I was back then, and how I lacked perspective.
Needless to say, I now know that real maturity doesn't come until eighteen.