Age regression can be to any age, including to baby, child, or teenager! it happens commonly because of trauma or mental illness, but can also be done by some people to de-stress or because they enjoy it.
In simpler terms, though, age regression is when you regress to a younger headspace, usually as a result or to cope with trauma or stress. However, like @/Pluralanomaly said, it can just be for relaxation. If you have any specific questions I'm happy to answer them :)