He is like a childish adult.

他就像一个幼稚的成年人。

youdao

It's time you put a stop to this childish behaviour.

这种孩子气的行为你该收敛了。

《牛津词典》

He is a childish man.

他是一个幼稚的男人。

youdao

So I realize that this was among a more childish demonstrations that I could come up with to illustrate this point.

我意识到这个一个很幼稚的演示,我可以举例来说明这一点。

哈佛公开课 - 计算机科学课程节选

We must stop thinking of space travel as a childish fantasy or a movie or television plot and recognize it as a serious project that may at some point become an absolute necessity.

FORBES: Space: Our Ticket to Survival

At a childish level, this is badmouthing a classmate and refusing to acknowledge his birthday.

ECONOMIST: Russia needs to play nice

This entire story originates from a childish feeling among Apple executives and fanboys that, if Bill Gates got a shiny badge, then Steve Jobs should get one too!

FORBES: Steve Jobs Knighthood Nixed By Political Pettyfoggery