I can speak to this subject with authority! I am the youngest 68 year old codger that you will ever run into. I have had one hip replacement and two knees that need replacing.
Most of us have reduced muscle mass. So, wedging is literally a pain. For this reason alone, many of us drop out after one lesson. I have found that stepping turns seem to work ok, providing the terrain is fairly flat.
Many of us have other limiting factors, such as limited range of motion in many joints. My knees have limited range. My femurs don't rotate as much as a thirty year old. By a long way. Dorsiflexion in my left ankle? What dorsiflexion? But most of us are not ready for the bone yard, yet.
Desire is the word. We have that and no one will take it away from us. As Nolo said in another thread, "there is more than one way to ski." What I suggest is teach the most efficient movements that you can. At the risk of starting a war, I "discovered" PMTS about five years ago. It rekindled the desire for skiing in me. Without it, I would have quit skiing altogether. After skiing, I had real significant pain in my knees and hips. But now, I can ski just about anything except bumps. I still have a delusion that maybe I can get my timing back.
Primary Movements is different than traditional teaching. There are many aspects of skiing that will require changing in order to be less physical. For instance, most skiers "force" the ski into a turn. By being patient and using the proper movements, the ski will do the work.
TTF, I hope this has helped. We old folks need all of the help we can get when it comes to ski instruction. Many of the younger instructors just do not understand aging. We need people like you that take an interest in us. Thanks.