If a non-Japanese entry is to be added, it must be one where the trope is discussed (25 is still considered young in many cultures; it should be lower for a non-Japanese entry to count such that it stands out).
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The below question was sent in from a 17 year old girl from Virginia.
Your parent just has to have some fun time to get things off their minds..!!
– Laura, 16 My mom recently started dating, and it’s really hard to adjust to.
Others just seemed to move the age up to match the times; "New Year's noodles" (referring to Toshikoshi soba, generally eaten on New Year's Eve) sets the upper limit at 31.
The Christmas Cake trope is related to Never a Self-Made Woman.
In order to fall under this trope, the woman must make a specific reference to the concept of "needing to have been married by a certain age" - typically by either having a complex about being older and unmarried, or making it clear that they don't believe in the concept of being "too old to marry".
(If the woman simply doesn't care, and isn't made to care by the plot, it doesn't count.) If the trope itself is brought up but not in reference to any specific female characters, that counts as an example as well.
And no matter how attractive a woman is, it's said, no Japanese male will want to marry her after she's 25.
Thus, "Christmas Cake" is used as a metaphor term for a woman rendered unmarriageable because she is past the freshness of her youth; roughly equivalent to "Old Maid." It's a common trope for a character to at least be surprised that another isn't married yet by this age.
If a character expresses this point in-story, he Likes Older Women.