I would take each client individually and assess their fitness level, health condition, and lifestyle and goals. Those over 50 can vary greatly in their abilities (I’m 48 and would like to think that at 50 I can still do all of the same things I do and enjoy now like boxing, HIIT, walking, and strength/circuit training) 🙂
It’s hard to give you suggestions for your client with bad knees when we don’t know much about his or her health history, and why the knees are bad. Is the client overweight or does the client have an injury or arthritis? Walking can be good, but so is swimming because it’s easy on the joints–and good cardio. Is there a reason you said no swimming?