Drivers will keep their tire pressure in optimum parameters, and make sure that the threads are still good, but most often than not, we overlook the other factor that tells us how good the tire is at holding the car on the road, its freshness.
As tires are made of rubber compound, and rubber tends to get stiffen over time, fresher tire will always yield better grips. As a rule of thumb, never buy tires that are manufactured more than 6 years ago, sitting on the shelves all these while.
Fortunately, the tire manufacture date is stamped on every tire as part of the DOT code. Look out for the 4 digit code (if it is 3 digit, your tire is already way too old, manufactured before the year 2000) stamped on the sidewall of the tire. The first two digits denote the week of birth, and the last two digits describing the year of birth.
In the accompanying picture, then, 3902 means this particular tire was manufactured on the 39th week of 2002. Dividing 39 by 4 (4 weeks a month) gets us 9.x, so it was produced sometimes in September, 2002. Simple as that.
So, next time when you put in new tires, make sure they are still new. If your current set of tires still has good threads left but are already over 6 years since it was produced, consider replacing them.
More info can be found on wikipedia Tire Code