Found this to be quite interesting:
Christmas Cake:
In Japan, age counts. Especially if you are a woman. The ideal of feminine beauty in Japan is youth and innocence. Plus, there's a lot of pressure on women to get married. So, if you're an unmarried woman, and heading towards thirty, we'd say that you're being "left on the shelf" or maybe "past your sell-by date".
In Japan though, they compare such women to a "Christmas Cake". It may well be sweet and delicious, but no one really wants any after the 25th. So, if you're an unmarried Japanese woman, after the age of twenty five, you're in extreme danger of becoming a Christmas Cake.
http://www.redbrick.dcu.ie/~melmoth/japan/c.htmlOkay, maybe this only applies to the ladies. I have to look for the term that describes the men.

Anyone is a Christmas cake?
