Definition of puerilism in US English:

puerilism

noun

Psychology
  • The state or condition of behaving like a child; childish behaviour or childishness; specifically (a) (in an adult) a reversion to infantile behaviour, usually as a symptom of mental disorder; (b) (in a child) the stage of normal development which follows infantilism and precedes puberty.

Origin

1920s; earliest use found in The Daily Courier. From puerile + -ism.