There isn't an "inspect the tyres fully and make each decision on a case by case basis" option... :nono
Anyway...
I would never use or dismiss any tyre purely on it's age. First and foremost is condition, then in no particular order comes intended use, size/weight of bike etc.
For instance, here is a tyre that was less than 2 years old according to the sidewall date when I took the picture and that was not the worst part. I sold the bike this tyre was fitted to, but managed to convince the purchaser not to ride it home - I trailered it for him instead.

Remember, that tyre was UNDER TWO YEARS OLD and had covered less than 500 miles. When I sold the bike a year later it was still holding air, but I'd say only just...
That's what improper storage does. I was never really bothered with the bike (as my K turned up at almost the same time, guess which I chose?) so it just got left sitting. I also think the owner before me must've got 'something' on it to accelerate the degredation as the valve stem had perished and split as well.
Now, on the other hand, we have this - do note it's the same manufacturer as the tyre above:

I took this picture this morning, about 10 minutes ago in fact. This is the NINE YEAR OLD front tyre fitted to my K right now. I use it happily with any load and at any speed (up to, but never exceeding the speed limit of course), it grips peg-scrapingly well wet or dry, it's not cracking up at all. Where exactly is the problem?
To my knowledge, I have not killed any nuns or kittens through my use of 'old' tyres.
Honestly, I don't think there is really a poll option I can choose unless you add the one in my first sentence.
Now, on a related but side note - Riding until you see threads..........
Seriously? Do you actually know any (two very naughty words) basket case that would ignore tyre wear to that extent?
Or were you extracting the urine?