Not childlike; inappropriate for a child to do or be(adjective)
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Use "unchildlike" in a sentence
"During its passing she lived through a wild, unchildlike woe of which she never spoke to anyone about her."
"'It would be better to make friends of the Salamanders than to force them,' he had written in his unchildlike hand."
"The child, and its unchildlike concerns, has been a leitmotif of what could be called the Iranian Ongoing Wave: to the joy of cineastes; to the annoyance of the proudly low-brow."