A few years ago I wondered the same thing and I think posted about it on this board!
Now I have an answer.
First off, though I LOVE diving (we moved to Bali to dive Indo) I have other loves as well -- they might ever be greater passions. Classical music, literature, hiking, nature, relationship with my wife, maybe other things. I am not one who "lives (only) to dive".
It would be HUUUUUUUUGE loss if we have to give it up. I am currently looking at back surgery at age 65, so I have had to contemplate it. My wife and I have had a few heart-to-hearts about it -- which is the wonderful part of having difficulties.
I have already taken losses to my activities in other areas: biking, hiking, backpacking, .... is diving next? Maybe.
What I do know is that we have reduced the types of dives we do: no more cold water dives (too much weight), no more dives without a DM to carry the gear. It could be that, as a result of my sciatica, we will have to eliminate dives where we put the gear on before getting in -- this would eliminate negative entries and higher or medium current dives.
What we have done also adds risk to the diving that we do: We don't have the same physical strength to struggle against current. If we had to come up the ladder with the gear on, it could be a problem. We could have a back spasm or a hip dislocation in the water. If we get lost at sea, we would likely not last as long as a 35-year-old.
We need to be more aware of how to evaluate sea conditions and whether it makes sense to sit out a few dives here and there.
We plan to dive as long as we can accept these risks knowingly, as long as it makes sense given our physical (and mental!) capabilities, though reducing the types of dives as we go. But when we stop diving, we won't stop living, or stop enjoying life.
We don't plan on living forever. One must take the losses that come with aging gracefully.
- Bill