The trick to buying a used car is balancing getting a discount on the new car price without buying something so old that you'll have to spend a lot of money on repairs. Most new cars today last longer than those built a generation ago. With that in mind, it’s generally best to buy a used car that’s 3-to-7 years old. However, keep the car’s mileage in mind, as high mileage can erode the car's value.