1 [in nouns] the people or language of a particular country or place:
Are the British unfriendly?
learning to speak Turkish
3 [in adjectives] typical of or like a particular type of person:
foolish behaviour (=typical of a fool)
Don't be so childish! (=do not behave like a child)
5 [in adjectives] rather [= quite]:
youngish (=not very young, but not old either)
From Longman Dictionary
Childish = meio infantil.
Mature + ish = quite (rather) mature .
Meio que maduro, um tanto quanto maduro.
However I guess the correct is "maturish"