My favorite definition of maturity is one I stumbled across many years ago that stuck in memory: "Maturity is 90% fatigue."
In other words, immaturity comes from the excess energy of your youth being applied to the wrong things -- or to two many things. Maturity means prioritizing properly, learning to care about the things that actually matter and leaving the rest to other people and/or God.
Therefore, to me, maturity ia the fulfilment of the serenity prayer: developing the strength to change the things you can, the fortitude to accept the things you can't and the wisdom to know the difference.