So what is maturity?
Cori's answer is flexibility, rolling punches, taking responsibility, being able to make decisions, and thinking about the consequences and repercussions of decisions. Understanding that life sucks sometimes and not having a temper tantrum and problem solve. You work through problems.
Cori also see maturity also as wanting to give back to the world because you see a bigger picture.
I think of it as taking a larger perspective. If it was up to kids, they wouldn't go to the dentist because of the pain. But adults take them because there's greater health in going to the dentist. We could multiply this example. We could also apply it to current world problems and wonder at how we are going to get our society to grow up.
I also think about patience, perseverance, and continuity of purpose. Mature people are engaged in positive goals, progressing forward in the world.
Cori thinks maturity is realizing you need other people. I would say realizing how important relationships are with others, is a sign of maturity.
Cori asks the question about personalities--is there one marker or markers that transcends personality, or is maturity more person specific?
Is there ever a time when maturity reverses? Is there a kind of ripening of maturity, like fruit, where afterwords it becomes rotten?
I think when you get older, you stop pleasing people, and get a little better at taking care of yourself to make sure you can take care of others, so sometimes people go overboard with that, and can get more selfish with age.
In some crucial way I think maturity is using your experience to help chart your way forward, using your experience in the world to approach it with greater wisdom and care. And to know about your experience, you have to face it. You can't avoid it through drugs, distraction, busyness or addictions that take you away from your experience. I always think of Marsha Linehan's saying the difference between someone who evolves is whether they face their experience. I also like the thing from Milarepa where he says something like a dog chases a stick, but the dharma warrior faces the stick thrower.
One of my friends said maturity was: "Subjugation of one's regressive emotions in the service of appropriate behavior."
Reading around the internet, the sophist think it's knowing one's role. According to a Sikh website, they think being aware, alert and not angry is mature. Dr. Gerald Stein from Chicago thinks maturity is humility, integration of emotion and reason, or head and heart, and finding solutions to problems, plus confidence and having a sense of what is worth fighting for. Also taking responsibility and not just going for thrills. Also articulating your principles and gratitude.
These seem like all good answers, with lots of overlap. You can do your own search, but it seems like there's a lot of room for different answers, and finding out what it means for you, and developing follow up questions can be an important process.