puerilism
/ˈpjʊərɪlɪz(ə)m/ /ˈpjɔːrɪlɪz(ə)m/Psychology and Psychoanalysis
noun
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.