Disregard. I stand corrected. They are not LEOs. Air Marshals work under the TSA, and are the "primary enforcement arm" per the website.
To answer your question about requirements:
Applicants must meet these qualifications in order to be futher evaluated in the TSO hiring process:
Be a U.S. Citizen or U.S. National at time of application submission;
Be at least 18 years of age at time of application submission;
Be proficient in the English language (i.e., able to read, write, speak, and listen);
Have a high school diploma or General Educational Development (GED) credential OR at least one year of full-time work experience in the security industry, aviation screening, or as an X-ray technician.
TSA job posting.