My understanding on the matter is that you don't want tires on your car that are more than 5 years old from the date of manufacture, regardless of mileage. From the factory there's lubricants that are embedded in the rubber to keep it soft, and those seep out over time and due to being exposed to the elements. So even if the tires aren't showing serious dry rot and only have a few thousand miles on them, that doesn't stop them from aging.
It's much like engine oil, really. Just because you only drove the car 1000 miles this year, would you not change the oil because you've still technically got another 9000 to go? I hope not, since the components of the oil will still break down just from sitting over time.
There were tires from 2002, 2003, and I think 2005 on my Boxster when I bought it a few months back. The car had basically sat on the lot where I found it for about 5 years and rarely moved. I actually drove close to a good thousand miles on those things until one day I happened to look at the right front (one of the 2002 tires) in the driveway and saw that the tread was literally separating from the carcass.
I didn't move the car again until I'd made an appointment to get on a new set of tires that were made at the beginning of 2010 at the tire shop less than 2 miles away. I'm sure they'll be long worn out and gone before 2015, the way I drive it.
But there's no way I'd ever let my tires go longer than that, even if they're still full of tread...to me it's just too much of a dice roll on old tires.