Recently replaced four tires on a RWD, full-size SUV which is driven frequently but always garaged when idle. Those tires had 2014 MFG dates on them. They had some wear but it had gotten to the point where I didn't trust them. Partially because they were 11 years old and partially due to wear. But, mostly, because it's my wife's vehicle. That was even though NONE had been on the vehicle since 2014. That just happened to be the last time those tires were made. I wanted matched tires and had replaced them, two at a time, more than once. (If it hadn't been for the matching-the-tires thing, the whole date issue would probably have escaped me, by the way.)
Never had an issue with tire failures when using those particular tires no matter their dates. I even kept the best one and it looks perfect inside. However, either the new ones have better traction, inherently or, as one poster suggested, the old ones had lost their grip. Quote from wife: 'When starting on a hill in the rain, the new tires never spin like the old ones did.' Hmmm . . .
New tires came from Sam's Club: Best overall price, great service -- With an appointment, even. Surprised (and pleased) the heck out of me. Balance was just right, by the way. (Ask for Chris . . . )
Editorial Comment: I despise arbitrary rules/policies like that over-five-year = NO MOUNT business. We have become a litigious, perfect-world, chicken-bleep society. Take the OP's case: Tires are well over that five year "limit" but, because they are already mounted, he is free to continue using them, indefinitely. Just as I was. And, as others have said, millions of 5+ year tires are on the road. Some have likely failed. So have newer ones, I suspect. However, if this really was a documented concern, I also suspect there would be laws in place prohibiting their use. Alternatively, I suppose it is due to two factors: Potential liability (litigiousness) and/or the desire to sell more tires. I am no fan of either.
However, you might want to visit our website:
www.mountyourown.tires . . . Yeah, I'm joking.