A big factor is exposure to UV light. If the tires are covered or the bike is stored inside, they will degrade slower than those stored exposed to sunlight. Another factor is temperature and also your riding style. I had a 10+ year old front tire on my Fatboy, but it is stored in a garage in SoCal with a cover (also it's a Harley and they are slow). I have since replaced it due to cracks on the sidewall. The rear tires are another story. If you are riding, they will likely wear out before they significantly age. I've gone through 4 or 5 rears to one front. Now that I have four motorcycles, tire usage will be reduced for each bike and I'll need to start paying closer attention to the tire ages.
Perhaps tire sellers came up with a plan to sell more tires by suggesting the manufacturer use a date code, based on accidents with old tires. Or it was driven by DOT. This started around 2000 with the use of a four-digit date code. There was a manufacture production code as part of DOT's tire identification number back in 1971, but that was not really used as a start date to determine age like it is now. (or maybe used during accident investigations). I've gone into America's tire with my 1995 300ZX and they won't fix a tire older than 6 years, even though corporate policy is no work on tires 10 years old.