replace or not depends on whether they are damaged.

It should not be determined by the milage of the car. You can test it by pushing each corner of the car down as hard as you can, and observe the rebound. A good shock should only allow it bounce back and stop there without repeated up and down motion. If it bounces up and down several times, the shock needs to be replaced. You can also check the shock for oil leak after you jack the car up. If there is any oil leak, that shock will fail soon. Obviously, if the spring is broken, it needs to be replace. If there is no problem, keep driving.