Boxster spare replacement

[mnc-i]

I think that is BS to a certain degree.
Yes, tires do deteriorate over time. But not as fast or as severe as a tire exposed to the elements.
Since your spare, is in the front trunk all you probably need is to make sure it doesn't have cracks in the rubber and it still holds air.
About three years ago I had a flat on my 1987 Corvette. The spare, carried under the rear of the car, had never been used or taken out of its carrier. It was however out of air. I took it to discount tire where they cleaned the debri from the wheel & lip of the tire and inflated it. No worries. It still is holding air.
I think someone wants to sell you a porsche spare tire.
Just my two cents...

[BYprodriver]

Sunlight is the main thing that ages tires. As previous poster said just inspect tire closely for cracks especially close to the rim. The tire is not "as good as new" but is probably fit for temporary use. Like any other tire do not exceed the capabilities of the tire & you will be fine if you need to use it.