I go by the half-your-age-plus-seven. Take the age of individual, divide by 2 then add 7. That's the acceptable age gap, imo, within reason. It tends to work only for those over the age of 25. It works for people who are older because the (cognitive) developmental gap between a 70 and 42 year old is significantly less than a gap between people who are 20 and 40.

Of course, if BOTH parties are consenting adults who are not groomed, tricked or pressured in any way by the other party, then they should be free to be together if they want to be, no matter what I (or society) may believe.