Nothing will make them last longer than keeping it out of the sun and rain. Most likely they are cracked to the cords and the cords have also rotted.
That said as long as you aren't plowing or doing heavy field work tires like that will last a long time. Its when you start getting bulges in the sidewall or a crack that starts to spread out that you need to open the wallet for new or different tires.
I've seen more than a couple tires that were being used for feed bunks that have found their way back onto tractors with the increase in tire prices.