By official, they mean a government issued picture ID. State drivers license, state issued ID in lew of drivers license (for those who do not have a drivers license), US Passport. Certified birth certificates are accepted for children under 18.
College and employer ID's are not acceptable.
My wife and I have changed to using our US Passports for airport ID's. For our children we carry certified birth certificates as they are only 10 & 11. We have not been asked for the birth certificates to date, but we've got them.