I've just been 'chatting' with some friends who wanted to go to Antarctica and they said they can't get insurance (not sure can't or prohibitively expensive) due to pre-existing conditions. They're still under 75, but getting close.
Qantas doesn't charge extra. It's expensive in the first place, but with no extra charge for a cruise, it has worked out the best for my upcoming Greenland cruise. (I have no pre existing conditions).