asking them to simply "not do it around x" isn't necessarily reasonable, because if it's actual age regression, it's not typically a conscious decision and therefore not purposeful. instead, it would be better to either see a therapist, as you suggested, or for the two of them to discuss what to do when they age regress. for example, if they sense that they need to/are about to regress, maybe they could confine themselves to a room (in the case where the two are living together) if they're comfortable with that. i also enter little space sometimes (nothing sexual about it, heaven forbid), and i don't typically actively decide i'm going to be little, but i'm well aware that i'm actually 23, and i could easily act my own age if someone was uncomfortable with it. for others, that's definitely not the case, because they regress to actually mentally being x age. i'm not completely clear on the terminology and stuff, even after some research, but age regression is typically used when the person mentally becomes x age, whereas age play is where you act/enjoy being x age but know you aren't really. (which is what i am, but i don't care for the term "age play" because there's nothing playful about it; it's very much a coping mechanism for stress/anxiety for me.)