There are multiple guys over 65 that work for my company, who don't seem to have an issue. You are having to pull quite a bit of weight while loading the patient, but the ramp system makes it very doable, and usually you have at least 4 others to help. The biggest issue I have had is the tight spaces. Having to lift 75lbs while bending at the waist, and repositioning yourself often includes getting over a seat to avoid wires, hoses, ect. that come with some patients. I believe it would be much easier in an aircraft with a big cabin like the PC-12 (my company uses PA31).
The most challenging part of the job is the limited time you have to plan the flight. 20 min wheels up time at some operations. Overall it is a lot of fun, but the flying is definitely very challenging. I would highly recommend you get very instruments current before applying to help training go smoother, as you are having to learn a lot about the medical side of the job.