The term may suggest to some that minors may be involved. That is just not the case. The regressive behavior of an adult or older child that resembles behaviour characteristic of an earlier stage of development. The derivation of sexual pleasure from acting or being treated like a baby. Whichever way you look at it all seem to agree that Infantilism and diaper fetishism fall into a category of phenomena knows as paraphilias.
Paraphilia is the modern day psychiatric term for sexual perversion.
According to the Diagnostic and Statistical Manual of Mental Disorders, Paraphilias refer to intense, repetitive sexual urges, fantasies or behaviors where the sexual goal is an unusual situation, activity or object. Well that conveniently covers most kinks I would think. It is unclear what causes a paraphilia to develop. Psychoanalysts theorize that an individual with a paraphilia is repeating or reverting to a sexual habit that arose early in life. Behaviourists suggest that paraphilias begin through a process of conditioning.