All bands wear out and degrade over time.
However, if you are seeing nicks and tears in your bands, especially close to the pouch, you are likely having fork strikes and are unaware of it. Upon release, the projectile is smashing the elastic against the frame of the slingshot, causing the telltale signs of fork hits.
One very common cause, however, is shooting rocks. If you are shooting stones from a band set that has a pouch made for spherical projectiles, it is quite easy to ruin a band set in just one shot!
This problem is actually easy to avoid and until you correct your technique, you will continue to damage your bands prior to wearing them out from use. Check out these videos for lots of info on shooting rocks.