The Brain
Maturity
Age
This is a large factor in maturity. I say this because the human brain develops over time growing and becoming more complex. This is because of experiences that you have every day.
How Do You Gain Maturity?
Well you already have. You see you as a person gain maturity from simply living life as you do. Let me explain, when you're brain is resented with a situation it hasn't had before it learns, it changes, it matures. So all you really need to do is learn and continue learning.
Signs of maturity
Commitment
Being committed to somthing is a strong sign that a person is mature.
Humility
It's often the case that a person that is mature person has experienced the feeling of humility 1 or more time.
Wisdom
When in a conversation is taking place a mature person will often think more about what they are saying and what they are going to say.
What's the importance of having maturity?
Here's why...
I have researched this question and it seems that the most popular answer is that it shows that you are trustworthy. If you think about it it kinda makes seance. When you are straight forward and have commitment and wisdom people tend to feel more open and reassured.