adult baby

noun1

1. •
   An adult person who enjoys role-playing a regression to an infant-like state and acting like a baby or toddler, as through wearing diapers, engaging in babylike activities, babbling, etc., generally for sexual gratification.

   • Well, he appears to be an AB. You know, an adult baby. It's a fetish in which the baby is taken care of in every way by a mommy or a daddy. — 2002