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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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2011-05-31T20:45:13.467
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2011-05-31T21:00:55.033
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numbers words readings
Why is 二十歳 pronounced はたち?
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](http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q117210981)
that explains the old way pretty nicely - including how to count hundreds,
thousands and tens of thousands!
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2011-05-31T20:56:14.790
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2011-05-31T20:56:14.790
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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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2011-05-31T21:34:48.520
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