My little take on the body, for us ageing folks...
Whenever you hear of anyone mention "exercise" we tend to think of exercise for the cardio system; however, after spending a lifetime of being an active person (I'm 54 and have been riding a bike as my primary form of transportation since my early 20's) and I also hike and do a lot of running. I've come to the following conclusion.
The cardio system is the easiest thing to keep healthy (unless you're one of the unfortunate, such as Bob Harper with genetic issues that affect the heart).
The hardest part about staying healthy is maintaining the musculoskeletal system, including connective tissues. I think the biggest mistake people make in this area is thinking that all exercise must be Low Impact. And I blame the fitness industry for putting this in our head in order to sell various types of exercise machines designed around low impact.
Whenever I participate in any activity/sport the limiting factor is almost always the musculoskeletal system, a good example is swimming, which I just started up about a month ago. If it were not for my arms getting tired I could go on and on and on, but my arms just start feeling light weights after so many laps, but my heart and lungs could keep the action up for much, much longer. Same thing when I go running, I get tired because my legs (or sometimes other things like my feet...) are done, but my cardio system is always ready for more.
And you can see this with the professionals, it's their musculoskeletal system that's the toughest to keep healthy, but when they can't race it's almost always an injury to their musculoskeletal system. You don't hear them complaining about cardio injuries.
So my recommendation would be to embrace the impact, impact is part of life. I think one of the best ways to start building up the body is thru weightlifting. Because it addresses every part of the musculoskeletal system and is a starting point for preparing the body for the rigors of life.
Not too long ago I was considering having to stop running, because of a nagging knee pain in my right knee, mostly a result of a childhood injury that nearly ripped off my right knee cap and damaged my patellar tendon.
But after reading and talking to others, I decided to really focus on my musculoskeletal system thru weightlifting. Long story short, what really helped my knee issues was to focus on strengthening my posterior chain, i.e. from my back down to my ankles. One of the best exercises I did for this were deadlifts and squats (squatting as low as possible for full range of motion, i.e. exercising everything).
The reason why this helped is because our quadricep muscles are exercised so much more than our hamstring muscles; this creates a muscle imbalance, which over time can cause serious problems and not every case will be the same and in my case with my pre-existing injury I was really feeling this imbalance.
No more knee problems, actually my knee feels stronger today than when I was 30, which is about the age I started feeling the knee pain and the doc told me I would soon need surgery -- over 20 years later and I'm not planning on any surgery.
We start losing muscle mass at around age 30, that's another reason weightlifting is so very important for maintaining health as we age.