Definitions

1. adjective

Lacking or not characteristic of juvenility; not youthful or immature.

“The unjuvenile tone in his writing was a deliberate choice to convey a sense of maturity.”

2. noun

Something that is not characteristic of or typical of juvenility; an example of an unyouthful quality.

“His unjuvenile behavior at the party was a source of concern for his parents.”