My tires are the originals. They were made in 2010. We have the trailer in heated storage for 5 months during the winter. I keep being told they are in excellent shape, no cracks or wear. Others tell ...
Replace them. I like Maxxis tires if you are going to stay with ST tires. As to whether you want to use LT tires, if you have a big heavy rig, it may be a good way to go if your wheels can handle the higher pressure, or you may wish to replace the wheels.