If you're middle aged (uhm, 40 yrs young and up) and your mattress is more than 10 years old, then it is time to get a new one. When you get older, your body's pressure points are different from when you were younger and your old mattress has the old pressure points; therefore, you are probably not feeling well-rested after a night's sleep.
If you're younger, then changing mattresses depends on whether it feels uncomfortable even after you've rotated it, you notice sagging or there are lumps, you can hear the bed springs squeek when you lie down or you have allergies and your mattress haven't been changed in years (there might be dust mites or bed bugs in it).