this guy is probably the best mechanic I've ever known it was a privilege to meet LouisI’m not sure if you have a discount tire but here they won’t even service older tires. They suggest replacement. Not sure way the cut off is however. But we get sidewall damage from UV commonly.
I recommend that anybody that likes their cars listen to his podcast http://www.agcoauto.com/content/news/p2_articleid/93