One of the sharpest changes (I guess a "point of inflection" it looks like?) seems to be at about 65, or the most common retirement age (here in the states at least). I wonder if there's a connection?

In hunter-gatherer societies, elders older than 60 have been observed as 1) looking healthy and 2) still being able to hunt, fish, trap, build, and pretty much everything else along with their younger counterparts. Perhaps at a slower pace, but they're generally far more fit than the modern world's old.

Pampered bodies grow sluggish through sloth, movement and their own weight exhausts them. - Seneca