Eleven and Twelve

Why don't we have "oneteen" and "twoteen" instead of eleven and twelve?  These curious words actually come from Old and Middle English words.  Eleven comes from "endleofan" meaning "and one more" or "one left past ten" and twelve is derived from "twelfe" meaning "and two more" or "two left past ten."  Note that two and twelve begin the same.

Whenever we think English has crazy exceptions, we can usually delve a bit into the origin to find meaning.   Standardized spelling is fairly recent, so as our tongues relaxed around certain words, pronunciation, and therefore spelling, changed over time.