Late thirties until you qualify for AARP (50), but honestly, we live an actively lifestyle for so long now that it really holds no significance anymore.
I am not questioning that you found this, but it assumes people live to be 120. The optimism is intense.
No, it does not assume that at all
“Middle age” has always meant the years from when you are at about half of life expectancy until you go to the next stage of life of newr-retirement or retirement
Aka 40s-60s