Re: Teaching injured or elderly students? Is Aikido for everyone?
Personally, I think if you don't know, it's better to simply be honest and open with people about the fact that you don't know rather than give them a false impression that you have answers you don't have. So if you don't know which things a particular person can do safely, if it was me I would let them know that you don't have the medical training or experience to answer that for them, but that you will be supportive of how they choose to handle it.
They may want to consult with a doctor or physiotherapist to discuss what movements are likely to be good or bad for them, or they may know their own body well enough to judge for themselves what they're OK to try. Ultimately it's up to them to take care of themselves.
IMO your responsibility is to provide an environment where their choices of what to participate in and how intensely are respected (or to tell them honestly if you don't believe you can provide that environment).