I felt that I had to post a reply for this subject since my hubby and I are what you refer to as "middle aged"! We have been dancing now going on 3 years. Our instructor only asks of us what we can do and she understands our capabilities that we bring to each dance.
We are not 25 year old competitors nor do we choose to be at this time. I think as an instructor you need to remember that not everyone can and will be able to do the dances as you would like them to do. My husband happens to be bow-legged and has had problems at times with certain dances however, he does the best that he can and our instructor is just as happy with his attemps!
We dance because we love it, it's something that we can do together, and it is exciting when we can do demo's and recital's for her and the studio that we attend. If you are concerned about liability in regards to this student getting injured then why not let him do what he can do with the range of motion that he has instead of trying to "force" his body to do what it cannot?
Now I am sure that you are a good instructor and you obviously care about your students but If I may, maybe not try to be so rigid in your teaching and just go into it with the idea that the experience is one of learning and enrichment!
Bobbie