I have heard 10 years as the hard limit, to as little a 6 from some sources.
If safety is your only concern you are probably good at 10 or until you see cracking, whichever is sooner.
If performance is your primary concern you will know without looking at a calendar.
Keep in mind too that it is the age of the tire, and not how long it has been on the car. Check all the date codes, they could be different.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked