Cars usually begin to show their age around three years old, usually the time when more parts need to be replaced. Of course, this still depends on how often you use your car and how you drive. Aggressive driving clearly wears them out much faster than defensive driving due to the amount of stress applied.
Car repair resource site AA1Car.com lists down the most commonly replaced car parts after a long period or set mileage. You can find some of these parts in the used car parts market.