An adult who has a fetish that consists of wearing diapers and acting as a baby is said to practice infantilism.
Infantilism is the practice of wearing diapers, baby clothes and using things commonly used by infants including rattles and baby bottles. This can be a fetish or even a lifestyle. Infantilism differs from a diaper fetish because a diaper fetish does not include acting like or being treated as an infant but is only about wearing diapers.
Often people into infantilism like to be treated as babies, including being bottle fed, burped, having their diapers changed, etc. Straight men who like to be treated as babies sometimes like to pretend to be breast-fed by sucking on their partner's breasts. Some people who enjoy the experience of being "adult babies" combine sex with an adult partner who often acts as the adult baby's caretaker as well as sexual partner. Many adult babies like the feeling of comfort and security they get by being taken care of. There are stores that specialize in plastic pants, diapers and all sorts of baby clothes for adult wearers. There are also websites, communities and even online personal ads for people who practice infantilism.