I totally agree that writing helps in mental maturity. Reading, writing and speaking all are related to the communication which means sharing of knowledge, feelings,thoughts and emotions. Sharing can improve your vision on any particular topic and it can enhance someones maturity level. Reading,writting and speaking are those type of skills which can change anyone's mindset.
Obviously writing which enhance our mental maturity. Conveying our emotions or thoughts by means of writing plays a prior role in the society to directing the people or government or even a writer to self analyse him or her which makes or enhance the mental maturity.