It really depends on context. My father is 9 years older than my mom, but the thing was when they got married he was 42, she was 33.
If I could be serious for a minute, I think there has to be a certain degree of emotional maturity for both parties. A 28 year-old dating an 18 year-old isn't wrong but it sure sounds odd, wrong or not. Sure, it's perceivable that an 18 year old can be wiser and more mature than his/her years indicate, but realistically it's just not that frequent to see. A big part of it is they haven't gone through the kind of life experiences that a 28 year old has gone through and during this span, it's arguably the most crucial life transition (high school into college into working life, family life, so on). With it, comes changes in lifestyles, habits, and perspectives. It's a different thing when say, you at 38 dated someone at 28 because more or less there's more likely to be a common foundational maturity level between the two.