You should train everyone according to their fitness level, assessment findings, physician’s clearance/directions, desired goals, using proper progressions, within your scope and abilities, and in manner that reflects respect and consideration.
I work with a group of 70 to 90 year old clients. Some of whom work harder than some of my 30 to 40 year old clients. One of my clients is over 70 and still competing in triathlons. I don’t make many decisions based solely on one aspect of a clients current status. And especially not be their age.