Actually, if it was following the pattern for most of the numbers between 10 and 20, it would be one-teen.
We get our numbers from the Germanic languages, which give the one's digit first (einundzwanzig = one and twenty = 21). The teens are different, though - (fuenfzehn = five ten = 15). Also, somewhere along the line, the numbers 20 and above reversed the order (Swedish and Norwegian numbers have this pattern, too).
Eleven is strange, because the Old English roots come from the words meaning "one", "loan". I still haven't figured out that one! The word itself dated from before 1200.
Now you know more than you ever wanted to know about 11.