Paraphilic infantilism is a paraphilia characterized by the desire to wear diapers, due to reasons other than medical necessity, and/or be treated as an infant or toddler. It is also characterized by the carrying out of these desires on a regular basis. Within the community of such diaper wearers, one who engages only in the erotic or sexual aspect of diaper wearing without experiencing any accompanying regression fantasies is known as a diaper lover (or DL). An adult who only engages in the infantilistic play aspect is known as an adult baby (or AB). An adult who may experience both of these things is referred to as an AB/DL. In 2004, an informal online survey of 80 adult diaper wearers showed that approximately 40% considered themselves strictly DL's, approximately 40% were mixed, and 20% considered themselves strictly AB's. The survey also indicated that there is a significant number of teenagers who engage in this practice. Findings of the American Psychiatric Association indicate that a majority of infantilists are heterosexual males.

DLs and ABs differ in self-image and the focus of attention. Neither includes a sexual preference for children. There is no singular, archetypical behavior for paraphilic infantilism, but a wide range of thought patterns and behaviors. Some fantasize about being free of guilt, responsibility, or control, whereas others might not. Some act indistinguishably from a baby at times, while others practice in a way that would probably not be noticed by passers by on the street. The desires and tastes of paraphilic infantilists vary around common themes of diapers and babyhood.

Age play

Ageplay or age play is a form of roleplaying in which an individual acts and/or treats another as if they were a different age. The practice can be regressive, in which the goal is to re-experience childhood, or sexual, recreating a sexual relationship with people of the pretend and/or true ages. Generally this can involve someone pretending to be younger than they actually are, but more rarely can involve assuming an older role.

Sexual variations may include among other things incest play, in which individuals recreate and sexualize roles within a family, and Daddy's girl fetishism in which real or imagined age differences are the basis of the roleplaying and the female is portrayed as the younger partner.

Ageplay is not considered to be related to pedophilia.

In non-sexual ageplay, people act as if they were a different age without the involvement of sexual acts, often with the goal of reliving childhood.