Is medical clearance a mandatory requirement for air travel?

No.

Well, if you are pregnant and appear to be near term, or have gaping wounds, etc., then you may...

um i've never gotten a medical clearance document.

unless you are seriously ill and say you require oxygen or something then yeah i think the airlines will require a dr's note clearing you to fly but healthy people do not need such documentation.

It really depends on what the medical condition is, and you didn't mention that to us. If you mention to your doctor that you need to travel by air and the doctor isn't concerned, there shouldn't be any difficulty with flying. If your doctor is concerned, and gives a list of things that must be done involving the flight, you can then contact the airline and ask the proper questions about possible restrictions in flying.

Well typically no..?

Unless you have some kind of condition where it is required. But usually you don't need medical documentation.

No, it is not necessary for passengers,however the crew need to take a test every six months.

Not unless you are the pilot.