Hi Rick. First, I’m not too sure that I would use YouTube (or any online) exercise videos as the guide as to how you should train your clients. As you know, everyone is different and so a “one-size fits all” answer here is probably not good. A LOT depends on the individual that you’re dealing with. It’s hard to generalize (and probably dangerous too) people by age or any other category. That being said, many baby-boomers have reached an age where orthopedic issues such as osteoarthritis in the knees is not uncommon. In my own practice I do not have my clients do deep knee squats no matter their age. This is my professional choice and I know that there are many, many trainers who do, and many articles on the benefits of same. However, there are probably an equal number of trainers who, like me don’t, and an equal number of research and other articles that suggest that this is not the way to go.
I say do your research, consider your particular clients and then make the decision for yourself – don’t rely on videos that are so abundant on the internet, some good, some bad.
I hope this helps.