Most active people over 35 or 40 have at least one physical problem that limits the exercises they can tolerate or that they should do. One of the values an expert trainer can offer is to provide work-arounds, especially after getting the advice info from a medical consultation.
For examples close to home, Mrs. BD had a bum knee, a meniscus repair, but still achy and unstable knee. Could hardly do body-weight leg exercises. After a year of carefully-graded and cautious leg strengthening, she is now pain-free with full mobility and is doing squats, lunges, and deadlifts.
In my case, a traumatic arthritis in one shoulder prevents me from overhead presses and push-ups. There is a knife-like pain which forces the effort to fail and collapse. It's too bad, because those are two excellent compound exercises but there are plenty of work-arounds for shoulder strength with slightly difference force vectors which are pain-free and so probably will not do any further damage to the joint - things like cable rows, lateral raises, and Farmer Walks.
Thus there are work-arounds for almost every frailty so a frailty is not an excuse to become frail.