Yes. Uncontrolled bruxism can result in significant bone loss and resiltant tooth loss. Think about how we loosen a fence post fro the soil ar a beach umbrella from the sand --- we rock it back and forth. This is what you are doing to your teeth. Not age specific. I have patients in their 20's show up with wear normally seen in 40 year old patients. You can still wear down your teeth well past 65.
Yes. I would say that 25-65 is the majority of your life and the answer is yes. Damage to your teeth and bone loss from bruxing is cumulative so every year counts. If you can control the detrimental consequences of bruxing you can worry less. Typically, occlusal guards are useful in controlling bruxism if there is not already damage to your TMJ and/or severe bone loss associated with your bruxism.
Teeth. Bruxism, per say wears out teeth. Painful mastication will cause bone loss, but because normal force cannot be exerted. This an indirect effect.