ませる

ichidan verb, intransitive verb

be precocious; be mature for one's age; seem grown-up

Describes a child who acts or seems older than their age, often with a slightly negative or critical nuance.

• あの子こはませているね。

  That kid is precocious, isn't he?

• ませた口くちをきく小しょう学がく生せいだった。

  He was an elementary schooler who talked like an adult.