The word "matureness" is not correct and is not normally used in written English.
The correct word is "maturity". Example sentence: His maturity in dealing with the situation was impressive.
Dictionary
matureness
noun
The property of being mature.
synonyms
Exact(1)
The search log of his English-Dari dictionary app read like a diary, toggling back and forth between ambition and despair: "rumor," "shielding," "reminisce," "minuscule," "cynical," "sock," "advocacy," "settled," "incredulous," "republish," "vampire," "apprentice," "rat," "moan," "madam," "phew," "matureness," "mature," "grownup," "awesome".
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com