I’ve had this car for a month and love it, but took it to my favorite mechanic who tells me one of the CV boots is worn. How does this happen?
If you’re just talking about the boot, it can crack from age, or road debris may have hit it and sliced it open. It’s basically just a flexible rubber housing.
In most cases, “torn” would be a better description of what happens to CV boots, rather than “worn”. As was already said, road debris is the usual cause. The potential problem for you is that the torn/worn boot may have allowed enough dirt and grit to contaminate the CV joint that it fails early.
The cost of replacing the entire CV joint and boot is not that much more than just replacing the boot, so you might want to consider doing this proactively, rather than replacing only the boot now, and then experiencing failure of the joint in a few thousand miles.
How many miles on your car? If within 3yrs/36k miles see what a Subaru dealers says. They are covered in the bumper to bumper.