Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
The TSA does not require any ID from 16-year-olds:
"Q. What ID is needed for minors traveling domestically/internationally?
A. Minor children (younger than 18) are not required to provide an ID at the airport security checkpoint. They will just need their boarding pass."
However, Allegiant requires that a 16-year-old travel with his/her birth certificate. Southwest requires proof of age (such as a birth certificate).