[Tor3]

For some reason many languages use an irregular system for numbers. Well, reasons are historical.. things may have made more sense in (e.g.) Old Norse than modern Danish, for example. Japanese though.. now that's regular. One, two, three, .. nine, ten, ten-one, ten-two,.. ten-eight, ten-nine, two-ten, two-ten-one.. and so on. And then 'hundred', and continue the same way. to-hundred-to-ten-five (225). The only stumble is when you pass 10000, as from then on the grouping is in tens of thousands, not in thousands. But with that the only issue there's only one struggle to master (for learners coming from a culture which groups in thousands).

Anecdotally, Japanese children learn to do arithmetic quicker than children from (e.g.) Sweden. But I've been unable to find real scientific confirmation of this.

[Ingaz]

I think every natural language is irregular when it comes to numbers.

And about Japanese: you forgot about counters for men, animals, flat things and so on.

You have ichi-ippiku-ippon-hitory - issai And absolutely fascinating "system" for "years old" - everything starts with -sai but 20 years is "hatachi"!

[Tor3]

Counters are a different story than numbers. The numbers are highly regular, with the tiny exception that numbers 4 and 7 technically have two variants, where in certain areas one is preferred more than the other, and the pronunciation of 9 when talking about time. But that's really minor.

As soon as you're starting to count it's different.

[boxed]

> I think every natural language is irregular when it comes to numbers.

But.. it's not. Japanese is not. Just because it's not metric with grouping doesn't mean it's irregular.

[samus]

Not being decimal is not the issue. The irregularity are special rules for certain numbers or ranges of numbers for which another pattern has to be used.

[boxed]

But... there is no such extra rules in Japanese. The grouping does NOT count as it is absolutely vital to make it possible to say numbers at all.

[samus]

A sibling commenter mentioned that there is a special word for "age of twenty"

[Tor3]

That's in the "counter" or ordinal number area. Numbers themselves are extremely regular: 20 is just "to-ten". The age of twenty is considered a very special occasion in Japan, and celebrated semi-publically (20 used to be when you became a legal adult, but now you can at least vote when you're 18). Thus that event is called hatachi, はたち, and is from the traditional number / counting system ("hata"=20). But it is also possible to say it as you would say 19 or 21, which is the normal number plus a "year counter" suffix - that depends on the focus.

The numbers (as in one, two, three..) are actually generally from Chinese, with exceptions (particularly where there are two options, as for four and seven), but counters (for smaller values) are generally from native Japanese and therefore sound different from numbers, and also changes somewhat with whatever it is you're counting. Japanese is so logical and well-structured that it appears that to compensate for that ease, you have counters.. (though we could blame Chinese for that as well, pre-contact there were only a very limited number of counter types (counter types are flat items, round items, long thin items, people, years..), post-contact there are hundreds (but fortunately it's possible to ignore a lot of them).

[ChainOfFools]

Japanese does have two different words for seven though. なな for everything that is not related to telling time, and しち when you're referring to units of time.

But this is a welcome trade for the regularity and simplicity of the rest of the number system. Although the flexibility available in European languages as described elsewhere in this thread does allow for texturizing things in a poetic or evocative way. being able to get two or three jobs done with one set of words is sometimes nice even if it does make things complicated when all you want is the most common usage of those words.