I am at that age now that Tension Based exercises looks better and better all the time. Let me explain. High rep calisthenics wear on the joints. Weights wear on the joints. I've been doing circuits with a variety of abdominal exercises with no more than 25 repetitions. However with any style of situp - Atlas, bent leg, etc. I'm still getting some wear on the lower spine.
Sooooo, why not do more of a Pilates style sit-up with the arms extended over the head at the start and then tense the abdominals strongly as you curl up. So anyway, we'll see how this works with sets of 10. Afterall, doing the tension style good mornings has worked very well for the lower back and hips. I had to move some railroad cross ties - weighing from 100 to 250 pounds - the other day, and I had no problems lifting up one end and dragging them. Some were light. Some were heavy. Two were very heavy. But, I had the strength to do it.
Hey Bob - I have a post on tension coming up - stay tuned.
I do not perform situps for ~1.5 years. Instead, I use 4 co-contraction exercises with a stick in my hands that affect the abs, low back, side muscles, hips, and shoulders.
1. Full range rotations to left and right at the waist level
2. Twisting
3. Bending to left and right
4. Crouching with the straight legs, touching the floor by my fists
I try to keep tension in the muscle antagonists through each set. It works for me. These co-contraction exercises can simultaneously develop flexibility and strength of the body's core.
Long ago a Coach mentioned using Planks instead of sit-ups for abdominal development & also Isometric exercise. Recently read that same idea in an article on using Planks but Isometrics were not mentioned.
All my ab work is done slow and I hold for 10 seconds at the hardest point. I'm also doing 10 reps. I actually like the results. I'd be interested how this works for You.