Based on the formula, a 21-year-old could date someone as young as 17, an 18-year-old could be with a 16-year old, and a 16-year-old with a 15-year-old.
Having a partner who is much older or younger has become somewhat trendy in today’s celebrity-obsessed culture, perhaps even more so among young adults.
The rule would theoretically create a paradox for people under 14, because the younger partner would have to be older than the older partner, but 14 is about the youngest you can be and make anything even remotely resembling adultish decisions respecting relationships anyway, so this rule works out rather well.
This has the advantage of allowing for a larger age gap the older the partners get; the four-year age gap between a 22-year-old and an 18-year-old is significant (and just barely allowed by this rule), but the same age gap between an 86-year-old and a 90-year-old isn't worth comment.
It's because of this urban legend that has gone around forever.