I'm over 60 years old and have shot knees also. I've been sailing a Megabyte for 2 years, and I love it. If you can move around the boat at a reasonably quick rate, aided by some anticipation of the need for the same, you will do fine. I chose the Megabyte after sailing a Sunfish, and it made my knees hurt, because one sits low on a sunfish which puts your knees very bent, which makes them hurt. On the Megabyte my knees are at a natural angle, and the only thing that makes them hurt is kneeling downwind, but you might be able to squat or sit on the side. My 240# self is too much "moveable ballast", so I must get in the center.
The boat is quick and responsive. The Zim folks are helpful, and there is a good bit of info on the class site. It takes a little bit of time to rig, but the mast is light enough so that I can do it all myself in 20-30 minutes. It's a great boat for Portsmouth racing, as it has a favorable handicap imho. I have the MKI rig and haven't had too much feeling of being overpowered, but I don't go out in winds much over 18 kts, but as I learn that probably will change. PM me if you have any more questions.