The lower back is definitely something I need to work on, though it's most likely more a hamstring and hip ROM issue (I've never been very flexible in either, and that has gotten much worse in the last 10 years).
The knees, it's not a leg strength issue - it's a knee joint issue. I've had issues with my knees since my teens. Some of my issues now are almost certainly because of poor choices in my leg training in the past. I had to give up distance running a few years ago because of my knees, and can't do heavy weights on squats (or anything with a squatting component) any more, because my knees just can't take the pressure on the lowering.
That said, leg strength does mitigate it some, and I'm better than I was a couple of years ago. I can still lift more than I expect, and can do relatively heavier weights on a single leg, using both legs to lower. But most of my leg training these days is just hiking the trails at work with my 30-lb. harness on, sometimes with additional equipment. Over the winter, I really hope to get back into the gym, if I can afford the membership.