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Wiktionary
ageplay

n. A form of roleplay in which the player deliberately acts an age different from their own (and usually younger).

Wikipedia
Ageplay

Ageplay or age play is a form of roleplaying in which an individual acts or treats another as if they were a different age, sexually or non-sexually. Ageplay is roleplaying between adults, and involves consent from all parties. Portraying any age can be the goal of ageplay, from babies, to the elderly. Usually this involves someone pretending to be younger than they actually are, but more rarely can involve assuming an older role.