(Adjective) Usually derogatory. Of conversation, thought, humor, etc.: befitting children rather than adults; childish, infantile, immature; unsophisticated; silly.
“The duke’s puerile love letters were read out, and he was mocked as a corrupt, lovesick old fool.”
Kate Williams, Becoming Queen Victoria: The Tragic Death of Princess Charlotte and the Unexpected Rise of Britain’s Greatest Monarch, 2008.