Designated trailer tires have special compound that extends their life while sitting exposed to UV.
General rule for light duty tires is 6 years, for commercial tires -10, but that differs between manufactures and the biggest factor is how do you protect them.
I just replaced tires on my utility trailer. Originals were 25 years old and still holding the pressure, but I figured out I got my money worth.
But I am religious about protecting my vehicles from UV.
I heard keeping tires in mud and snow shortens their life as well, but I moved to CA to not have this worry.