"Mature and composed" is one way to describe a piece of writing that is calm and considered in its language and tone. This expression could be used to describe a piece of writing that shows a level of professionapsm, insight, and poise.

Here are a few example sentences using this phrase:

"The report was written in a mature and composed manner, presenting the information clearly and objectively."
"The speech was depvered with a mature and composed tone, reassuring the audience and inspiring confidence."
"The email was written in a mature and composed style, addressing the concerns of the reader with tact and diplomacy."

"Mature" means showing a level of experience and understanding that is expected of someone who is older or more advanced in their field. "Composed" means calm and controlled, with a sense of poise and self-possession. Together, these words suggest that the writing in question is thoughtful and measured, rather than rushed or impulsive.