5-7 years seems to be the recomended range from tire mfg for ALL passenger car/trailer/MH tires. In our area, many dealers will not do any repairs (nails etc.) on tires over 5-7 years old.
I don't worry about our trailers to much since we put 8K miles/year on our trailers so in 3-4 years they need replacing anyway.
As a side note, I've noticed that most passenger car tire tread wear warranty is for XXXmiles/3-5 years. So much beyond a 40K tread wear warranty doesn't do most people much good.