Take a peek at this link to decode the DOT stampings on the tire:
http://www.tirerack.com/tires/tirete....jsp?techid=11
Fine hairline cracks are fine, but I would start shopping for new tires. Generally, you want tires no older than 6-10 years for "optimum" performance. Vary quite a bit with brands and tire compounds and your area.
Since people go by mileage, it is hard to say exactly how long tires can last. If you live in a hotter climate (lots of sun), example states like TX, CA, AZ, NV, NM, and FL - 6 years might be all your get. In cooler weather states like MN, WI, MI, ND, and MT - 10 years might be the usable "safe" lifespan of tires. Of course, this assumes that they were properly maintained.
I've had some tires show signs of dry rotting after two years, have a set of Continentals that are probably 16-18 years old but look like mint condition - but don't drive on them for safety reasons.