What's the deal with date stamps on tires?
 

is it really true that they somewhat 'expire' 6 years after the date stamp?

i.e. the date stamp on one of my tires is 1508...so that means, the tire was manufactured during the 15th week of 2008.

however..when someone buys a used car with original tires, those tires can be super old...

how paranoid are you? is it just a marketing gimmick? or does a tire truly
start to disentegrate after the 6 years?

There are factors other than age and tread depth.

If the rubber is old and hard, the outer layer will be cracking and provide terrible traction. I've seen 2 year old tires that are hard and cracked that I wouldn't feel comfortable driving on.

My gf's sister asked me to sell her winter tires a few years ago and they still had 11/32nds of tread left. When I felt the rubber, they were hard a rocks. I checked the date code and they were made pre-2000. I couldn't sell them in good conscience.

Most manufacturers recommend changing tires after 7 years (due to rubber losing its elasticity). If I recall, the government says a tire "must" be replaced after 10 years, no mater the condition/wear.
Some manufacturers offers "unlimited warranties" on their tires (Pirelli's P4 has an Unlimited year/135 000km warranty), but I have no idea what happens if you have such tire and they start cracking.
Long story short, a quality tire can last more than 6 years, but the rubber lose its elasticity and it's not recommended to keep em more than 7 years.

Why take a chance when the tires get old?

Aged Tires: A Driving Hazard? | Video - ABC News