I turn 50 and read "Ageless" just to find out that sometime between 50 & 65, the immune system just about disintegrates- thymus (maker of killer T cells) disappears, lymph nodes also start to disappear, cellular trash collection services begin discovering trash they can't metabolize, kidneys & liver filter out less trash per round of filtering.
Good news is the science on the biological mechanisms has gotten pretty good?
https://www.amazon.com/Ageless-Science-Getting-Older-Without/dp/0385544928