I've been teaching functional movements to a diverse population for three years now at Crossfit North. I'm also a disabled veteran with fairly extensive damage to my lumbar spine and associated nerve damage.
Stuart McGills' book "Ultimate Back Fitness and Performance" is a gold mine of good information. Steve Shafleys' summary is good, the concept of "qualifying" athletes (everyone) for movements is a very effective approach, and "Neutral Spine" is a concept which everyone should understand and implement.
Teaching people, young or old, healthy or broken, to implement the neutral spine position is a natural complement to teaching proper hip function. Insisting on the maintenance of the neutral spine posture also makes it very clear whether an athlete should be doing a certain movement or not.
This approach has worked for stabilizing and managing my back issues, allowing me to perform at a reasonable level, it has also been effective for many of our clients, including some older and some badly injured people. I don't avoid sit-ups or any movement completely, but I do have some people modify or avoid certain movements based on their observed ability to maintain proper body position.