Some tires last, some don't. I have some on yard equipment that are over 50 years old, it is not uncommon to see original WWII tires on trailers. I own over a dozen trailers and just replaced a set because the tread started coming off, they are about 12 years old. I have seen cheap tires that only last a few years. Generally speaking ST tires are junk compared to LT or P tires. Inspect your tires and take reasonable care to keep them inflated properly for the load they see. If someone wants to pick some number and change their tires at that interval and it makes them feel good about themselves then good for them but age itself has little bearing on if you should randomly change them or not. There are fine articles on tire inspection and care on the net, read them rather than pick a random number and go by that. Rotate your tires and use the spare on a regular basis. Don't let the trailer sit too long in one place and drive it, sitting is bad for tire life.
Using large diameter quality tires well under their load capacity and inflated properly for their load should last a very very long time. Just inspect them, but research what you should be looking for, some cracks and things that look like defects should not be of concern.
If someone can tell the age of tires on the side of the road as they are driving by them you must have eyes that would make an eagle jealous.