There is no specific age difference in dating that is universally regarded as taboo, as views vary significantly depending on the culture and other circumstances surrounding the couple. One rule of thumb regarding acceptable age differences in dating is that an adult can date anyone within half his age plus seven, although there's no evidence to support this rule.
While studies have found examples of relationships with either gender as the older or younger partner, it is more common for older men to be in relationships with younger women, as of 2015. The average age difference is typically only about two to three years. One explanation for why men prefer younger women is because they find younger women more physically attractive. Women, on the other hand, prefer men who are successful and established in their field, which is more common among older men. There tends to be less of a taboo for older men to date younger women than older women to date younger men.
The "half your age, plus seven" rule was also a calculation used to determine the ideal age for a man's bride, before becoming a rule of thumb regarding socially acceptable age differences in dating. A 2000 study in "Evolution and Human Behavior" found that men and women were both more conservative regarding the minimum and maximum age they would date in comparison to the rule.