I don't believe all May/December relationships are automatically icky... but too many of them are. Sometimes people genuinely love each other, age difference be damned, and it's not just some predatory "December" gettin' some hot, young "May" tail. As previously mentioned, Kevin Kline and Phoebe Cates are a good, healthy example.
I take issue when the younger party in question is either underage (ew) or of legal age, but only just barely (minor improvement, but still ew). When the younger party is a tried and true adult, I say there is absolutely no harm, no foul. C.S. Lewis was nearly 20