Maturity is the ability to control anger & settle differences without violence & destruction.

Maturity is patience.  It is the willingness to pass up immediate pleasure in favor of long-term gain.

Maturity is perseverance, the ability to sweat out a project or a situation in spite of heavy opposition & discouraging setbacks.

Maturity is the capacity to face unpleasantness & frustration, discomfort & defeat, without complaint or collapse.

Maturity is humility.  It is being big enough to say, "I was wrong.  I am sorry."
And, when you are right, you need not say, "I told you so."

Maturity is the ability to make a decision & follow through.  
The immature spend their lives exploring endless possibilities & then do nothing.

Maturity means dependability, keeping one's word & coming through in a crisis.  
The immature are masters of the alibi.  They are conflicted & disorganized. Their lives are a maze of broken promises, former friends, unfinished business & good intentions that never materialized.

And based on the Serenity Prayer:  
Maturity  is

the art of living in peace with what we cannot change,

the courage to change what we know should be changed, and

the wisdom to know the difference.

LA Times June 17, 1993  Ann Landers' column.