For one thing, bone is not merely a solid, inanimate object. A bone lying in the desert is a bone, but a bone inside the human body is living tissue. It is not dead, and it is not rock solid. In fact, it is quite porous. A bone is like a sponge. A bone is living, and it grows or breaks down, depending on circumstances. A bone gets denser when you use it and gets more fragile when you avoid using it.
Nowhere in the human body does bone rub on bone in a normal, healthy human being. A bone is protected by synovial fluid. Ligaments, muscles, joints and bones are well lubricated. If you've ever had bone rubbing on bone in your body (and there are situations in which people have such terrible nutrition that their body breaks down and doesn't rebuild all the supporting tissues and fluids, so they can end up having bone rubbing on bone), it's not a problem of use; it's a problem of poor health. It's a problem of eating lots of processed food and animal products and avoiding raw fresh juices and healthy nutrition. It's a problem of having such a terrible lifestyle that the body even begins to attack itself, resulting in autoimmune disorders, such as arthritis or type-1 diabetes.
Any kind of joint pain, bone pain or bone arthritis has a cause. It's not just that things wear out, and anyone who says the human body is like a bunch of car parts is using the wrong metaphor. They are flat-out incorrect, and they do not understand how the human body really works. Car parts wear out the more you use them because you have gears rubbing on gears, which creates friction and the breakdown of metal, and, eventually, you end up with a gear that's worn out. That does not happen in the human body. You don't have bone rubbing on bone in a healthy human body. The more you use your body, in fact, the healthier your body gets. The body doesn't wear out with use. You don't just have a certain number of heartbeats in your life and then you're dead. In fact, if you exercise your heart, you get stronger. If you give your heart nutrition -- through B6 vitamins, calcium and magnesium -- your heart gets stronger. If you exercise your muscles, they get stronger. If you exercise your bones, they get stronger, too.
The human body is a miracle-class machine that adapts to stress by repairing itself. The more you use it, the stronger it gets. That's why you have people in their 70s, 80s or even 90s out there running or jogging up a mountainside.
You can prove that what I'm saying is true by asking yourself, "Who are the most diseased people I know?" Once you have those people pictured in your mind, ask yourself, "What is their level of physical activity?" You will most likely find that the most diseased people are those who don't engage in physical activity. They are the sedentary people who don't use their bodies. That's who is diseased. They are the ones whose bodies are wearing out. If bones wore out through use, people who didn't use their bones would have the strongest bone density, and people who did use their bones would have the weakest bones. Of course, that's the opposite of what happens in the real world. In reality, people who don't use their bones have the weakest, most fragile bones, whereas people who do use their bones have the strongest bones. This is a very clear example of the inaccuracy of the scientific theories and metaphors of Western medicine.
The healthiest people in society, in fact, are those who avoid doctors and instead heal themselves through nutrition, Chinese medicine, acupuncture, massage therapy, chiropractic care and so on. The least healthy people in society are those who take lots and lots of prescription drugs, who visit Western medical doctors, who ignore nutrition and who think health is just a matter of luck. These are the least healthy people in society. It's actually very clear, when you look around, who is healthy and who is not. It's very clear which medical theories are consistently true.
I can give you many examples of ways in which the theories behind Western medicine or conventional medicine are utterly false. There's really not even honest science behind most of Western medicine, because virtually all the science has been distorted and selectively reported. Take an informed look at what's really going on in the medical science community, and you'll find lots of selectively designed studies and outright fraud, because a lot of the sciences are basically sponsored by drug companies.
As wrong as they are, however, these inaccurate metaphors (like "your body wears out with age") exist in a pervasive way among the practitioners of Western medicine. Metaphors such as the human body being a system of parts are undoubtedly pervasive, and you will probably continue to hear these types of metaphors in the future, so be aware of them, and know that any time a doctor or a physical therapist says that the human body wears out with age, they are either simplifying the explanation for your benefit or they are simply misinformed.
In reality, the human body is a self-healing system. Bone is living, breathing tissue, and the only way to have healthy bones, joints, ligaments and tissues is to use them. Give them space (through stretching, yoga, Tai Chi, etc.) and nourishment (through circulation of nutrient-rich blood). Use them on a regular basis and you'll be active all the way up to 100 years of age. You won't live forever, but that's because we are actually genetically programmed to expire at some point in order to free up resources for future generations (it's true!), but you will undoubtedly live longer than most, and you'll live far happier.