Many people belief maturity as one of the pre-requisite before marriage both as a lady and a guy. Even thou maturity is not in age, but every body sees it from different perspective.
I see maturity as d following
1. CONFIDENCE ON YOUR ACTION /DECISION: When u can stand on ur decision without being ashame, irrespective of d opposition. Ability to say No many times without feeling guilty
2. SELF CONTROL: U are mature when u can control everything about yourself, your desire, spending, emotion, likes, what u say etc.

Let us hear your own definition of Maturity

Walking tall, thinking positively, seeing potential in all things, understanding that you know but nothing, having a mentor to look up to, socializing with people both in the real life and online, talking less and listening more, and the ultimate, being a man, and not a boy!

In addition to you and big_t's point.......................

Living your own life.

Doing unto others what you want them do unto you.

Doing the right thing at the right time.

Growing up and remaining up

Maturity = SELF-CONTROL + DELAYED GRATIFICATION.
ABILITY TO RATIONALIZE THOUGHT PROCESSES BEFORE TAKING ACTIONS.