Good advise from all. I run a tire distribution business. The truth is, their is no real data to go on for when a person should replace a tire soley due to age. With a tire being made or many different chemical compounds it does have a "service life" and that is affected by many factors including weather, vehicle weight, speed, inflation, and road conditions to name a few. The best advise is the have them inspected by a reputable shop, with people you trust to deal honestly. Some of these still do exist. It's hard to get the whole story without getting the car off the ground and inspecting the whole tire, not just "taking a look". The number one tire killer is low inflation. Keep your tire inflated with the manufactures recommended pressure. Check them often and address any balance issues quickly. Most modern tires are much better than what we had even ten years ago. Hope that helps.