You have cells that help build bone (osteoblasts) and others that help break it down (osteoclasts). While it’s true that production of osteoblasts can slow down with age, the way you use your body continues to exert an influence on the formation and resiliency of your bones. This also holds true for your joints and muscles, as well.
Your posture—while standing and moving—creates either virtuous or vicious circles, meaning that as your posture affects your body’s health and appearance, so will those factors then influence your posture. This is why practice doesn’t always make perfect. Doing the same thing over and over reinforces that particular behavior, regardless of whether it’s beneficial to you or not.
An obvious example of this is the hunched-over posture that’s encouraged by the use of computers, phones, and modern chairs (ugh, especially car seats). As you regularly adopt this posture, certain muscles lengthen and others shorten which pull on your joints and bones in various ways. Over time, your bones will begin to take the rounded-shoulder shape and you may even develop kyphosis (excessive upper back rounding, aka a “hunch”).
If you’ve noticed this in your mirror, all is not lost. You can change your posture.