It is not considered as a disorder as per the DSM. Age regression can be an aspect of trauma or DID or other disorders, but not in the context that OP has described. It is not a regression of a few years (as typical with children experiencing regression in response to something) and is done by choice as opposed to DID which is outside of the person’s control. The real difference is voluntary vs involuntary. Voluntary can be used as a means or relaxation or other things- but it is of one’s volition to engage in it.

They are engaging in regression- they are not ‘unable’ to behave otherwise.