When should an outdoor above ground oil tank be replaced?

The tank should be marked as to thickness. Should be at least 12 gauge (lower the number the better). I don't know much about 'guarantees' (there always seems to be a loop hole) but you should check with your insurance company to make sure you're covered.
Besides x-raying a steel tank there really is no way to tell when it may fail, although there may be some indications or warnings.
The biggest killers of steel tanks is water. They rot from the inside out. If the tank is properly installed, pitched toward the bottom outlet and either draws from the bottom or someone annually draws some off the bottom to check for water, then you are on the right path. Throw a bottle of additive at the beginning of the season with a fill up, something that disperses water.
I've seen water kill a new tank in 6 years, and I see 40+ year old tanks with no leaks.
They make drip pans for steel tanks so at least you'll see some oil before a catastrophic failure.
Usually there is staining at the seams, or you see a small drop attached to the underside of the tank. That usually indicates a pinhole, and there's never just one, so don't touch it/wipe it/poke it. The rest of the pinholes are being blocked by sludge.

Tanks rarely let go unless they are over pressurized, the gravity supply line leaks or gets damaged, or someone rams it with a vehicle.

If you do replace it, double wall tanks are much better.