The ball joints in your lower arms will last a long time if they're kept free of dirt, but if the protective boot cracks they'll start to degrade. Unfortunately, the rubber boot doesn't live forever. This replacement boot can be installed easily to replace a cracked one, and may keep you from having to buy a new ball joint.
If your stock boot is damaged, check to see how freely the ball joint moves. A good ball joint will be stiff, a worn one will move easily.
Factory Mazda part (H266-28-448). Fits all 1990-05 Miatas.