Geo,
Answer I think has to be somewhat geared to tire used. Tires I used are 12 PLY, NYLON LOAD RANGE F, MAX LOAD 1765 LBS @ 110 PSI COLD,TOW-MASTER,20.5x8x10.

Tow speed: I run up to 75 with these. Total boat and trailer weight was about 3200 lbs, so tire weight rating gave me some room to breathe.

Age? 3-5 years max, but wear was not so much to tread as it was to the sidewalls that would start to show cracks. That told me the rubber was getting older and brittle and that meant I was risking a wall failure. So when they starting showing significant cracks, they got replaced.

Mileage: I never really tracked it, I went by tire tread depth with the Lincoln head penny depth guage, and if tread got shallow, I dumped the tire to reduce the possibility of puncture damage.

Wear patterns: If the tire is no longer round and tread depth varies you may have either a frozen set of springs, or a misaligned axle. Tread that wears more on the outside or inside of tires points mainly to misaligned axle...my experience.