Why is 二十歳 pronounced はたち?


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?




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!




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!

