My last partner was 20 years older, but very much the little boy, and I the "daddy," which made for some interesting dynamics.
And while I did like being the 20 year younger daddy, I missed the feeling of being taken care of myself by a source of strength. And as a gay boy who wasn't very connected w/ my real "daddy" and didn't get all the daddying I probably needed, there was probably a similarity/familiarity w/ my older partner that fit the model. Needless to say, the relationship ended.
But, I wonder if the relationship w/ our real dads kinda has us interested in getting and living out what we didn't get as kids - to feel a strong male nurturance..? To feel taken care of by an older "daddy"? Maybe this sounds obvious, eh?
Plus, there's a power dynamic at play between younger and older guys. Youth is so desirable for many reasons (more libido, an older man's longing for youth through association, a substitute for a son, mentorship, generativity, etc) that the younger person has power for the older; the younger of the two also experiences this position of power and might feel more secure that he is in the driver's seat in the relationship, so there is less fear of being rejected or abandoned by the older partner, thereby making it kind of a win-win relationship in that respect..?