The reasoning behind this designation is that while one person is young and in the "Spring" of his or her life (thus, "May"), the other is in "Winter" (ergo, "December").
However, there are many instances, as in the case of my own clandestine affair with the most wonderful woman in the world, where the age difference is either inconsequential or isn't even a considerable enough gap to constitute a May-December Romance.
Ashton Kutcher and Demi Moore - This would be the "etc." I suppose... Something just isn't right... o_O.
Me and Her - She's only four and a half months older than me, but I suppose I just like the scandal that comes with calling it a May-December Romance. =D. Oh and it's not really clandestine anymore. I just persist to call it that for the same reason.