I'm 53 and got my first clipless pedals about three years ago. There is no going back. Being attached to the pedals just makes pedaling much more efficient. The attachment is more secure than toeclips and straps, yet is easy to release. Straps used to make my feet go numb when I cinched them tightly. Clipless is easy to work with, just click in and go. Quick twist and you are out. The pedal stroke can be nice and round by being able to pull up on the pedals as well as push down. I've been through SPD, Time ATAC and two types of Crankbrothers eggbeater and candy pedals. I like the candy pedals a lot. Good float and easy release with good feel to them.
Now, for knees, seat height and setback must be set right on. Also, the bicyclist with knee problems must be told to watch their knees to make sure they go up and down while pedaling without having any side to side motion. Part of knee pain for some riders is seat position. Part for others is due to improper cleat placement on the shoe. For others the issue lies with knee rotation while pedaling. It takes someone who knows about the physiological aspects of bicycling to analyze a rider to determine what the root cause of knee pain is.
In my case (and this is just me and is not applicable to someone else, necessarily) I need one cleat to be slightly behind the ball of my foot due to leg length differences. I also need my seat height and setback set perfectly. A couple of centimeters too high and I'll have knee pain and a couple of centimeters too low and I will have knee pain as well. Too high, pain in front, too low, pain in back of the knee. This is a rule of thumb. I have a bad knee from spending my life compensating for differences in leg length while marching and walking straight. If I had just let myself walk with a slight limp I'd probably have less knee problems today, but that's neither here nor there. There are many experts in proper fit that understand proper positioning to protect knees. Clipless pedals are part of the equation.