Serving Economies through Aviation and Technology in the 21st Century (SEAT 21) Act - Authorizes the Secretary of Transportation to allow public charter operators, if certain conditions are met, to provide air service to small communities utilizing smaller jets (turbine-powered aircraft with nine or fewer passenger seats). Authorizes airports to choose not to receive such air services (opt-out) if they notify the Administrator of the Federal Aviation Administration (FAA) not later than 180 days after making their choice.