I've seen trailer tires so bad that you could see the belt through 3/16" gaps in the tread. And they still rode just fine, just wouldn't want it behind my vehicle.
Oldest I probably used was the spare on my '85 F-250 I used to own. I had a belt separation on the regular tire and had to mount the spare, which I later discovered was made in 1993. This was in 2013-ish, so twenty years old. Oddly enough it exhibited very little signs of cracking (probably from being mounted underneath the bed all that time away from sun), but it was definitely hard rubber. I had no money then, so drove on it a couple months till I could afford a new set of tires. It was a P235/85/16 IIRC.
These days I don't really go by date, but when the cracking gets bad enough, it's time to go.