Here is what I have learned from reading different tire threads over the years.
Look for and find the manufacture date on the tires. 4 digit number specifing the week and year the tires were made. Usually on side wall of tire with an oval around it.
Tires loose approximately 10% of their rated carrying capasity each year. This is important if your normal loaded trailer weight is near the rated capasity of your tires. If you travel light and have plenty of capasity not being used, it becomes less important the first few years.
Check before every trip for bulging, cracking, spliting, uneven wear and proper inflation. First sign of any of these, replace your tires. When they blow, the often do much more damage to the trailer wheel wells, floor and any systems close to them (water tanks, sewer pipes, flooring or structure) than it would have cost to replace them.
Protect your tires from direct sun while in storage will help maximize their life. Also avoid using tire shine products as they are usually bad for the tires due to having protroleum products in them.
Expect to replace at least every 4-5 years if you take very good care of them, more often if not.