Personally, this is one of those questions where there is no right answer. People are attracted to different things, some people fall in love with those much older and those much younger than them, some can make it work, some can't. My only issue is the power dynamic (though age isn't the only thing that can cause it) which may lead to the relationship becoming unequal. Instead of a relationship between two people who love and respect each other, we have a relationship where one person (man or woman) is taking advantage of the other.
Basically: if someone falls in love with someone else (regardless of age, looks, gender, etc) and they respect them as an individual? I don't have a problem with it. If the only reason they are dating is because of age? Midlife crisis, pedophilia, whatever, then I have an issue.