infantilism

A state of arrested development in an adult, characterized by retention of infantile mentality, accompanied by stunted growth and sexual immaturity, and often by dwarfism.

Marked immaturity, as in behavior or character.

An infantile act or remark.

Arrested or retrogressive development; the persistence or recurrence in the adult of those characteristics which are normal, but transient, in the child.

An emotional dependency on being treated as an infant.