No offense to anyone but let's be honest...if you're still doing the same things in your late 70's that you were doing at 55 then you weren't really doing all that much at 55.
From my own circle of friends, I ride mountain bike with a 75-year-old guy who can keep up with me on climbs and leave me in the dust on descents. For reference, I am in my early 50s, ride 6 days a week, run once a week, lift weights, do martial arts/sparring and hike, and yet he can still keep up with me.
I know another guy who is 83. He rides a popular mountain bike trail around my neighborhood. The trail is 12 miles with 1,300 ft of elevation gain. He rides it EVERYDAY. We jokingly call him the "Old Man" but he is still faster than a lot of the young people on the trail.
I have another friend who is 70 and rides 4 times a week and does dirt jumps. Great rider.
Sure, many people are living longer because of the miracle of modern medicine, but many older folks are also fitter than ever well into their 70s and 80s and doing physical activities once thought impossible for their age.