Crunches and sit-ups can cause injuries to your spine and neck. You can end up with back pain or exacerbate an already injured back.
Crunches are not particularly effective at getting rid of belly fat, despite the infomercials selling you easy sit-up solutions. They do work great on trim, fat-free bellies!
Instead, do planks (properly) and workout your core without crunching your back bones and muscles. You need to strengthen core muscles to protect your back. Planks are great for this.
The last thing your aging back and knees need are squats. They are great for your glutes and legs, but your 40 year old back will pay.
Aging knees are pretty fussy about excessive bending, especially when weight bearing. Save your knees and try to avoid knee replacements by avoiding squats.
Squats build hip muscles and make you look heavier. Add a normal tendency to deposit fat on middle aged hips and squats may add to the appearance of fat.