I just learned the word 喜寿 here on WaniKani, meaning 77th birthday. I went to a monolingual dictionary hoping to get an explanation of why the 77th birthday is so important to get it’s own word (no less from the kanji for happiness and longevity), but it just gave the definition.
I found this article that explains how ages used to be counted long ago (数え年), gives other special ages, and explains how to celebrate. But as far as I can tell, it doesn’t explain why these ages deserve special celebration. I found another article that seems to explain the origin of the original cursive character for 喜寿 (which now that I’ve read the description, I realize it was explained in the Goo dictionary entry as well, just more concisely), but as far as I can tell it doesn’t explain the origin of why 77 (and the other ages) are important.
Does anyone have prior knowledge of this? Or could someone with a stronger ability to read Japanese help me search for an answer?