I've ridden on tires that old. No problems yet.
I also got a durometer to test them since I've got bikes I put new tires on six or more years ago but hardly rode. They've all been out of the heat and sunlight so haven't aged particularly, but it's nice to know just how hard or soft they are. So far, no problems - and I ride in the rain regularly. The last set on the RS were ten or twelve years old Metzelers and they finally wore out - but the last trip was 800 miles to Santa Cruz CA for a tech day. Not a problem, except for getting down to the wear bars sooner than expected.