It's recommended that tires older than 5 years be disgarded regardless or mileage as the rubber compounds age. But who really does this? 48 or 60 minutes TV programs did an entire story on this. I see by the way you spell Tyres... that you are probably not in the united states.
Tires have "wear bands" built into them.. as your tread wears down you'll see this band horizontally across your tire. This means the tread is gone and it's no longer safe to drive on the tires.
I can recommend you visit and online site called TireRack.com if you want to learn a lot about Tire codes.