With reference to a person who is no longer a child: not befitting maturity; puerile, foolish, silly.

figurative. Of persons: Immature; of crude or hasty character.

Childish.

spec. depreciative. Unduly childish; babyish, infantile; immature.

Made babyish or childishly foolish.

Characteristic of or suitable for a baby or young child; infantile, excessively childish; unsophisticated, silly. Also: resembling (that of) a baby…

Usually derogatory. Of conversation, thought, humour, etc.: befitting children rather than adults; childish, infantile, immature; unsophisticated…

Said of the face. Puny, childish, baby-like.

Puerile, childish; also, naïve.

Having or showing an emotional or intellectual development not befitting or characteristic of a mature person; juvenile, childish; silly.