Age play: When one or more partners act a different age than they are. For example, in DDLG (Daddy Daughter Little Girl), the "little" acts like a young girl. Some refer to this as "age regression." Both people involved are always adults.
Ageplay can be mildly or very sexual. Ageplay can enhance the way partners feel and make them more comfortable in their role within dominant/submissive relationships. In a BDSM ageplay relationship, the roleplay could include some sort of punishment for "bad behavior", such as spankings, that both consenting participants agree to.
Usually sexual roleplaying involving Daddy/daughter, Daddy/son, Mother/son, Mother/daughter, Police/arrested, Teacher/student, Doctor/nurse, Doctor/patient, Uncle/niece, etc.
The main stipulation is that one person is either older or playing older than the second person, or the submissive person is younger or playing younger than the dominant.