The Brief: Age regression is a mental state that someone may retreat to in which they act, think, and/or feel like a younger person.
Not to be confused with age play, which is a kink, age regression is a non-sexual psychological state in which someone mentally regresses to a younger age. Some people may engage in platonic CGL relationships as a part of their age regression.
One person can engage in both age play and age regression, but the two concepts remain separate. There is debate about the exact definitions of these terms, which can vary from person to person.