Why is there "ninety-nine", & "twenty-two", but not "onety-one"?

Answer 1

"Eleven" goes back to Middle English "en-leven", whose first syllable is a relative of "an/ane" meaning "one", and the "tw-" of "twelve" gives away its connection to "two". So we can sort of see that these two have something to do with the system of counting by ten. In fact, the original meaning of these two words was "one left" and "two left" (after counting to ten).

Here's how it happened:
"What about the anomalous eleven and twelve? Why do we not say oneteen, twoteen along the same pattern as thirteen, fourteen, fifteen? Eleven in Old English is endleofan, and related forms in the various Germanic languages point back to an original Germanic *ainlif, "eleven." *Ainlif is composed of *ain-, "one," the same as our one, and the suffix *-lif from the Germanic root *lib-, "to adhere, remain, remain left over." Thus, eleven is literally "one-left" (over, that is, past ten), and twelve is "two-left" (over past ten)."
http://www.thefreedictionary.com/dict.asp?Word=eleven

Finally, the reason "eleven" and "twelve" did not end up simply conforming to the "teen" pattern is that these were common, well-established forms. And it is precisely the common, everyday words that are LEAST likely to submit to "rules". (That's why the "irregular" past tense verb forms -- had, were, went, etc-- and irregular plural forms --men, mice, geese-- are almost always found with simple, common words.)

Of course, there were some alternative forms out there. We still use an alternative word for twelve -- "do-zen" itself shows us that (compare German "zehn")-- though it has gained its own special use (for a GROUPING of twelve). In fact, the fact that many things were divided into units of twelve also helps explain why eleven and twelve were treated differently. (Note that the use of twelve has ancient roots. See for example the Mesopotamian mathematical system, which used both six [and its double twelve] and ten --and gave us our 12 hours, 60 seconds, etc) That is NOT to say that these number names originated in counting by twelve, simply that the later USE of twelve contributed to the names' not being changed.
http://en.wikipedia.org/wiki/Duodecimal#Origin

Answer 2

It's because the numbers between ten and twenty follow a different (and inconsistent) pattern.
From the way 13 through 19 are named, you would expect 11 to be called something like "one-teen" and 12 something like "two-teen." So go figure!
But someday, when I become Emperor of the Universe, I will rectify this.

Answer 3

onety is ten ( 10 ) therefore onety-one is eleven (11)

Answer 4

In my opinion people earlier counted small numbers. Then they had to invent larger numbers and they did it easier. But it is only my opinion :D

Answer 5

Someone didn't thought of it.