Why is it "teen" instead of ten? Where did the word "teen" originate? When you say "sixteen" you are obviously saying six and ten. How did it become six and teen? I already saw the post on "Why do eleven and twelve get unique words."
Teen (from www.etymonline.com)
word-forming element making cardinal numbers from 13 to 19, meaning "ten more than," from Old English -tene, -tiene.
The Invention of the Teenager (from www.ushistory.org)
In the 19th century, the American world consisted of children and adults. Most Americans tried their best to allow their children to enjoy their youth while they were slowly prepared for the trials and tribulations of adulthood. Although child labor practices still existed, more and more states were passing restrictions against such exploitation. The average number of years spent in school for young Americans was also on the rise. Parents were waiting longer to goad their youngsters into marriage rather than pairing them off at the tender age of sixteen or seventeen. In short, it soon became apparent that a new stage of life — the teenage phase — was becoming a reality in America. American adolescents were displaying traits unknown among children and adults. Although the word teenager did not come into use until decades later, the teenage mindset dawned in the 1920s.