Agreed, even 5 years is pushing it, but if they have good tread and proper inflation and the sidewalls are protected while it's in storage, then you might be okay. I think the average suggestion is every 3 years, if they are unprotected from the elements, per the Discount Tire Company place mat at their counter. I had a similar story on my used Roo that i bought 2 years ago. The owners said that the tires were in good shape and a quick glance didn't tell me much more until about an hour into the trip and i blew one of the tire sidewalls. Come to find out, they were all under inflated, had dry rot from sitting in water while it was in storage, and the sidewalls and inside the tread area was cracked. My mistake for pulling out of their driveway like that, but I learned a thing or two about tires...but the hard way. I wrote up a sticky a while back about tire care and storage and all that good stuff. Here's a link to it - Advice on tire care
I wound up replacing the small 13" wheels and tires on my roo to a set of 14" wheels and tires and that was the best thing I ever did. I've towed that trailer around now for almost 500 miles and not a single problem (knock on wood).
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP