Age regression is when somebody reverts to a child-like state of mind, often as a coping mechanism for things like PTSD, depression, anxiety, and other mental health issues. Basically age regressors are more at-peace and worry-free whilst in "littlespace" (A term for when one is in said mindset). These people simply need breaks from the stress of being old and often have a childish personality even outside of regression (Though sometimes it's not as obvious).
Contrary to popular belief, age regression is NOT a kink or even related to sexual intercourse whatsoever. Some age regressors refer to themselves as littles and to their significant other as "Daddy", "Mommy", or just their "Caregiver" Since they'd typically take care of their "little" as if they're an actual child.