There is a simple test you can do on tires:

1. On a new or fairly new tire, depress the tread with your thumb and notice how pliable and soft it is. It should be soft and pliable and there should be no cracking.

2. Do the same test on your old tires and compare results. If still soft, you're probably reasonably safe for moderate speeds that don't heat up the tire as much as high speeds.

If the tread on the old tires is quite hard and rigid, the ride will be more harsh and will make more tire noise. And if there are also cracks, failure is more likely than if it were soft with no cracks.

The temporary spare in my 1989 daily driver's trunk is more than 27 years old. It's still soft with no cracks. No plans to replace it.