infantile
adj.adj. behaving in a way that is childish or not mature. You use this to describe someone who acts like a small child, often in a way that is annoying or silly.
adj. characterised by or suitable for an infant; lacking maturity or sophistication. Often used to describe behavior that is perceived as childish or inappropriate for an adult.
His infantile behavior made it difficult for him to work with others.
The manager was frustrated by the infantile arguments between the two junior staff members over a minor mistake.
The critic argued that the film's infantile plot was a deliberate attempt to appeal to a younger demographic, though it failed to resonate with older viewers.
Mid-15th century, "pertaining to infants," from Latin infantilis (“pertaining to an infant”), equivalent to infant + -ile, from īnfāns. Sense of "infant-like" is from 1772.