What is the requirement to become a flight attendant?
There is a (relatively) new requirement for Flight Attendants (on aircraft with more than 20 passenger seats) to be certified as per the FAA. This was done in the "VISION 100—CENTURY OF AVIATION
REAUTHORIZATION ACT", in section 814 if you want to read it.

The applicable CFR is 49 U.S. Code § 44728 - Flight attendant certification. In summary it says:


You have to have a certificate of demonstrated proficiency, this looks like a pilots license.
Demonstrated compliance with the air carrier approved training program.
Proficiency to read, speak, and write English
Comprehend written material in English
Ability to write incident reports and log entries/statements
Carry out written and oral instructions


Now, the actual physical requirements are up to the airline, as long as the individual can complete the training program, they are physically allowed to become a flight attendant (physical appearance requirements aside). As for "legally" allowed, they need to be able to pass the same TSA background checks that pilots are subject to. So legally, almost all airlines require:


U.S. Citizen or appropriate work visa
Be able to enter/exit the United States without any issues (no visa problems with other countries, although for regionals this may not apply)
Be able to pass a TSA/FBI background check
No DUI or felony convictions within the past 10 years


There isn't a lot more "legal" to it than that, they don't have to demonstrate the use of safety equipment if the airline's training program doesn't require it.