Time and time again, every study on longevity finds that simple activity is insufficient for good health and fitness into old age. There is something special about subjecting the body to resistance or weight training. Indeed, the cultures whose constituents have incredibly long healthy life don't just do some more walking, Tai Chi or Yoga. They carry water pails. They pull up fishing nets. They till gardens. They work fields. They teach contact Karate. They do physical labor. They subject the body to resistance stress.
This is getting studied definitively now: http://news.wfu.edu/2017/10/31/lose-fat-preserve-muscle-weight-training-beats-cardio-older-adults/. The unique stress of additional loading against the skeleton, the integumentary system, muscle systems, and even the brain and nervous system is different than going for walks or hiking. It's not just different. It is better.
Art de Vany has been working to define exactly what "aging" is. And in his tireless search, he's found no obligatory system which presses us to age. There is no genetic time clock. There is no predestined need for it. Rather, it's just accumulated damage paired with the tolerance of this declining ability. When we don't get up and do something difficult, we are reinforcing for our bodies that they SHOULD deteriorate. With deterioration comes increased risk of damage. With damage comes age. With age comes the fallacy that we should "take it easy." And so on.
So, this doesn't mean you have to get a gym membership. But you do have to contract muscles hard in a way that tells your body it needs to get toughened up in preparation to dominate the normal demands of daily life. Otherwise you are going to accelerate aging and degeneration. I would argue that the gym facility affords many people a better way to do this, because the progressions can be meted out in an as-tolerated manner. That is, someone may really only be able to handle a 2lb dumbbell for a given exercise for a single set of 2-3 reps. With the artifice, the structure of a facility with this equipment, one can take a very strategic approach to modestly but progressively subjecting herself to ever-increasing demands in a slow and controlled fashion. In a few weeks, that same person may handle the 3lb dumbbells. She may handle two sets of 5-8 reps. And so on.
Given that people require ease and convenience, I think it may be an unrealistically tall order to ask from the populace to get a little more serious about their heavy physical labor in a not-so-controlled environment, like hauling hay bales and some intensities which are far more unpredictable. Ergo, you must train with weights.