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二十歳 is a (to me) bizarre exception to the usual number+さい rule for discussing age. Is this rooted in 20 being the Japanese age of majority?

Added: To be more specific: why isn't it pronounced にじゅうさい like the rest of the さい words for age?

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There are several (amusing) theories here. gogen-allguide.com/ha/hatachi.html I'm not really qualified, but will post a rough translation if no one does. – Louis May 31 '11 at 20:56
にじっさい is used as well and is correct. – repecmps Jun 5 '11 at 2:19
does it seem like i'm from the olden days if someone asked for my age and i replied にじっさい ? – Pacerier Jul 10 '11 at 14:35
@Pacerier maybe it's just me, but it does. I'd say にじゅっさい. – syockit Aug 26 '11 at 18:37
@Pacerier はたち is the usual reading (though it originally referred to the count 20), にじっさい/にじゅっさい is the other. Actually, your にじっさい is the correct other reading (as slated by Modern Kana Usage), and writing it as にじゅっさい was not correct until review of Jōyō Kanji in 2010. Originally, 十 was read as "jip", written in old Japanese as ジフ. If connected with 歳, that'd be "jip-sai", so writing it as "じっさい" is closer to that sound. As to why it makes you sound old, it's because you are pronouncing it correctly and younger people who don't know better assume it's "Juu"+"Sai". – syockit Aug 27 '11 at 10:34
up vote 22 down vote accepted

The はた there is part of the same series of Japanese readings for numbers as ひとつ、ふたつ、みっつ and so on. Where the ち comes from - that I do not know. It also makes an appearance in some other common words, such as 二十日(はつか), although in a slightly mangled form.

There are readings for the tens after that as well - for instance 三十(みそ) makes an appearance in words such as 三十日(みそか) and 三十路(みそじ). The rest of the tens are formed by adding そ to the corresponding "ones" stem: よそ いそ むそ ななそ やそ ここのそ.

Although rarely used these days, the old way of counting was quite flexible. Here's a Chiebukuro question that explains the old way pretty nicely - including how to count hundreds, thousands and tens of thousands!

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Wow, I honestly thought this question didn't have an answer and it 'just was' pronounced that way. Shows how much I know! – Ali May 31 '11 at 20:58
Thanks for this excellent explanation about where the はたち reading came from. I'm particularly interested in why we still use はたち and not にじゅうさい, so I've added to the question. – sartak May 31 '11 at 21:02
Probably because 20 is coming of age; words that are treated with special significance often refuses to change. – Nate Glenn May 31 '11 at 21:03
Thanks for the awesome article link! I didn't even know it was possible to count above 10 using Japanese numbers... – Kaji Mar 17 '14 at 23:14

Some theories from http://gogen-allguide.com/ha/hatachi.html

Please forgive and correct any mistakes I made.

Theory: はた means 20. For example: 二十歳 はたち、二十人 はたとり、二十年 はたとせ。 ち (個)is a counter for the ひと、ふた、み counting system.

Theory (folklore): The 旗乳 (はたち)folktale. During the Warring States period, a young soldier who turned 20 years old wore a banner (旗 はた)of his lords family crest on his back into battle. On that banner he put 20 decorative things (乳 - ち) to match his age. So the theory associates the age of 20 with being old enough to risk your life at war, an adult.

Theory (folklore): If you count your fingers and toes you end up with 20. Deriving from 果て (はて), you reach the end (はて) at 20.

There are more on that site.

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