Maturity is a big word which in general (there are always exceptions) comes with age. I believe there are some key aspects of the emotional and psychological maturity that are in particular crucial.
The first came to my mind (not a reflection of my personal situation) is handling of frustration and disappointment in life.
As one grows older (I wonder why I say that), one will inevitably face frustration and disappointment at some point of his/her life. The ability to bounce back and move on from those moments is of utmost important. Yes, don’t dwell on it too much. Learn from the experience and next. Chop-chop.