Ok, granted but I've just come from AA web site and on their children traveling alone page there is a link "documents required" follow that link and all it says is (as copied and pasted from the site).
"Identification Requirements
Passengers 18 years of age or older are required to have government-issued photo identification for travel within the United States. However, you may be required to present proof of age (such as a birth certificate) at the airport for any children under the age of 18 who are traveling with you.
Passengers under the age of 18 who are traveling outside the United States are subject to the same travel document requirements as adults. Visit the U.S. State Department's website for information on
passports as well as
visa and immunization requirements for international travel."
Nothing about domestic flight ID requirement.
I think they do this on purpose so they can get an extra $100.00 out of the parents.