Average life of tires is about 6 years. I replace all my tires every 6 years from the date stamp on the tires and I store my boat in my garage so the tires never see the sun when not in use.
Make sure you have proper tire inflation for the load, but to be on the safe side, just pump them up to max cold psi, you're probably close to the max load anyway. The speed you drive also has a big effect on the lifespan of the tires. The faster you drive with under inflated tires the more likely you are to suffer a blowout.
There are also great aftermarket tire pressure sensors you can put in your trailer tires that give you the temperature and pressure so you might detect a potential blow out with rising temps in the tires.
http://www.orange-electronic.com/en/...ofit_p409.html
When I put new tires on my trailer I had them put the TPMS in. I also upgraded the tires to Kumho 857 load range D tires, stronger than the load class C tires that came on the trailer. I now have a small display in my tow vehicle that lets me know what the pressures are and the temperatures of the tires, it's great peace of mind.