NO this isn't common.
OK, maybe with preteen kids throwing their bikes around, crashing them, leaving them in the drive where they get run over it's more common.

If it's truly 5 mo old - It is a warranty issue, AND they need to replace or realign the derailler hanger or it's going to happen again. I would insist on LBS fixing it under warranty as most likely it was an adjustment/set up problem. Even low end Shimano stuff generally "works" even with no to low maintenance. Most of my bikes are 20+ years old and get thousands of miles a year and all are original deraillers. Of course they are a bit higher up the food chain and get regular maintenance.

If you really have to buy it yourself, a Tourney can be had for about $12, and an upgrade to Altus for $20.
Without getting into pull ratio discussions the derailler doesn't care about speeds, the shifter takes care of that.

http://www.treefortbikes.com/product...d-8-Speed.html

http://www.amazon.com/Shimano-RD-TX7.../dp/B001606M9G