All ST tires are suspect 3 years after their MFR date, regardless of what the outside tread looks like. Most tire stores will tell you to replace them every 3 years (of course, they sell tires...). In my experience (I've had 6 blow outs in 10 years) they just don't last more than 4 years from the MFR date, I NEVER had one make it to 5 years.
I have had the same experience with Carlisle and GreenBall (Transmaster). Yes, they are cheaper, but you will need to replace them more often. You probably should have replaced all 5 when you blew the first one.
I'm now running Bridgestone Duravis R250's, they cost more, but I'm hoping to get more life from them (about 5 years would make it worth the cost). And of course, no more blow outs or trailer damage!
Good luck with whatever decision you make.