When doing the evacuation drills to ensure people can leave the plane in 90 seconds, are they tested with disabled people, ones in wheelchairs or, like myself, people who have difficulties walking and use crutches? I would never be able to jump on the slide or even evacuate in 90 seconds so what would that mean for me? Would I just be left behind?
As @mins said in the comments, the 90 seconds is a limit for a evacuation test. Having equal test requirements means that all planes have a minimum, similar evacuation capability.
That said, the conditions of an evacuation test are rather benign: people participating in them know nothing is really wrong, there are no hazards like smoke or fire, and the plane is in a normal attitude (no broken gear or hull, engines stopped). See.
In real emergencies, the time to evacuate may be greater than 90 seconds, and the crew and ultimately the captain has the responsibility to ensure that everyone has been evacuated safely and leave the ship the last. So, crew incapacitation left apart, you wouldn't be left behind.
That is a very good question, and from my experience with flight test, I would say that I would guess no. All of our crews had FAA Class 3 medicals. I would have to research the certification requirements.
With that said, there is increasingly a spotlight put on the airlines when passengers with disabilities are mistreated.