Half your age plus seven is an unofficial rule of romance often credited to French author Max ORell (Léon Paul Blouet) in his 1901 love manual with the incredibly romantic title of Her Royal Highness Woman and His Majesty Cupid.

Is it OK to have a 7 year age gap?

According to the rule, the age of the younger partner (regardless of gender) should be no less than seven more than half the older partners age.