Allow me to clear up some misconceptions.
First, tires do age in storage, but much, much more slowly than when in service. For practical purposes you can ignore the date of manufacture, if you know the installation date.
I have seen data that says that properly stored 3 year old tires are indistinguishable from freshly made tires. In the tire industry, it is believed that even 6 year old tires can be sold as new, but I know of no data to support that (except for the 3 year old data)
When it comes to the ethics of retail sales, if the customer wants tires within a range of manufacture (say no older than 1 year), then that should be brought up BEFORE the sale takes place, not after the tires are pulled from the shelves, and certainly not after the vehicle is at the house. In this case, the OP wanted tires within 6 months and he said it up front - GOOD!!.
The sales guy said he couldn't do that. At that point the negotiations were over. The choices were 1) to go somewhere else, or 2) accept the conditions at Costco. It is not ethical to try to find away around the conditions.
Lastly, tire aging depends on ambient temperature. Tires age much faster in Phoenix, than they do in Minneapolis. The best way I can describe this is that if you live in AZ, TX, NV, CA, and FL, 6 years is the limit, and if you live in WI, ID, ND, MN, and MT , the limit is 10 years - and states in between are …. ah ….. in between.