I haven't watched the video in the first post, but I am reading the book in the second post, which is very informative. Peter Attia is not into bs. He takes a view from both a medical and scientific background.
Basically it's true that ordinary average sedentary Americans (and Brits!) lose tons of muscle mass and strength in old age. But if you keep on exercising, strength training etc and eat a healthy diet then muscle loss and decline in aerobic fitness (VO2 max) can be minimised. Maintaining metabolic health appears to be the key to growing old without all the usual associated diseases.