Conventional wisdom - including from tire experts - says that 5 years is about all you can expect from trailer tires. Appearance is not a good incidcator, and it is common for good-looking tires to fail. Just ask me - I have done it several times.
I would suggest that you rummage around the Internet for info specific to trailer tires. Roger Marble is a good source, since he has no ax to grind. By contrast, tire manufacturers will either tell you that their tires last a long time, or they may avoid the issue entirely - they want you to buy their tires. And tire shops may overhype the problem, for the same reason - they want you to buy new tires from them.
So take my comments with a grain of salt - but do your own research to confirm or deny it.
Bill
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