Well, for me, a lot depends on the type of riding involved.
My antique Kz900 has Metzelers on it that have low miles but which are at least seven years old now. I just use this bike for knocking around on; no high speeds or aggressive cornering. I think it's safe enough for that, even though the tires are old. Provided I don't subject them to sustained high speeds or other forms of loading, such as carrying a passenger, etc.
I physically check the tires for signs of cracks and so on. But I don't plan on replacing them until the tread is worn out enough to justify doing so.
Now, for a sport bike that's ridden at high speeds, or cornered aggressively, I would not feel comfortable using a tire older than three years, irrespective of the mileage on them.