In my humble opinion, I would never go beyond the 5 years of use from date of manufacture on an ST tire. Just not worth the risk of a tire failure, do to the damage it could cause to the trailer.
I know a LT tire or car tire should be replaced between 6 and 10 years after manufacturing. With 10 years being the maximum life span of a tire, if this is a daily used tire for driving.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67, TDR 145K+miles