How long should tires last before they dry rot?

I think you are reading dates codes right. But even in a state that has a lot of UV rot, that is excessive. And you said they were covered? Well you can't fix them...Time for new.
Last bunch I bought was Goodyear "wrangler" so far no problems other than one valve extender keeps wobbling around, going to use RTV on the valve stem threads and see what happens.