I used the 4 year number as what I'd consider a good rule of thumb in most cases. Sure, tires will hold air for longer than that, but that does not mean they'll reliably do that running 70mph either. Given the mileage from the previous bias ply style tires, which we usually wore out before any age issues came up, that was about 4 years of normal driving back then. In that time frame too, many large companies had their fleet vehicle tires swapped out at no more than 35,000 miles instead of letting them wear longer (probably due to decreasing performance with decreasing tread in some driving situations).
I believe the tire people have already spoken. First the article in Old Cars Weekly a while back. Then what I found in the '56 Studebaker Golden Hawk Registry newsletter "56J". And then the statement about "6 years" from the BFG representative that has been quoted more recently.
As I mentioned, what tires a person buys for their vehicles is their own judgment call. Everybody has their own orientations about reproduction "period" tires, more modern radials, or somewhere in between. It's your money and your vehicle and your decision.
In the "56J" newsletter, I found mention of even bias ply tires coming apart due to age issues. It seems that no tire construction type is immune to that situation, just that we typically didn't encounter it in the earlier times when the cars were newer and we bought tires sooner (due to tread wear-out).
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