(unless you have chronic pain from a relatively young age)

I have, which is what got me into the gym in the first place. I just didn't know what I was doing :D

Most of them are probably still around.

Yes. Averages tell the story. Average mortality age in Oz is 82. That is a lot of years of pain, if not preparing for health.

Eldest and I have tried stressing the importance of also working joint strength and flexibility

Has he not learned anything from gymnasts? :D But yeah, most young men don't care about the reality of the future.

I've been a bit slack at work and haven't been jumping on and off the boxes as much as usual.

Sounds like you are working for Australia Post ;D