Three major reasons why Comair any many other regional airlines require their pilots to be ATLEAST 21 years old.
1. Comair (and most other regionals) listed a college degree as preferred. Most people cannot get a 4 year college degree by the time they are 21. So this is used as a way to weed out people who don't have college degrees.
2. Like rickair said, passengers don't like to have teenagers (literally) piloting their aircraft. While I know a few 19 and 20 year olds who have been hired at regionals, they prefer not to hire pilots at that young age.
3. They want you to be able to upgrade when your seniority allows it. Again, you need to be 23 to upgrade to captain, so at many regionals like Skywest, ExpressJet, Republic, etc..., hiring a 19 year old means they will spend 4 years as FO.
Get your college degree. Then go fly for an airline. You will be cheating yourself out of MILLION(S) OF DOLLARS by not getting a college degree.