Most of the manufacturer recommendations I've seen have been for annual (professional) inspections starting at 7 years old and replacement regardless at 10 years old. Such an inspection would require the tire be dismounted to check the inside as well as the outside, and I suspect the cost of doing that would make it tempting to just replace them at that point.
Needless to say, if there are deep sidewall cracks or other non-repairable damage (bubbles, tread separation, broken belts, that kind of thing), the tire should be replaced regardless of age...and likewise if the tread is worn to the wear bars.