It's more than 10 years old and sports lumps, dips, or trenches.
Your hips sink too low, so you wake up with a sore lower back. Sagging means the springs are worn, the batting has flattened, or the mattress or box spring was poorly made.
You wake up stiff or with shoulder or neck pain.
You've changed: You've gotten much lighter or heavier, which can affect the body—mattress fit.
You've gotten older. Young folks tend to like firm mattresses, but as the body grows more mature, it requires more cushioning, especially if you are a side sleeper. Incidentally, getting a good night's sleep generally gets harder as you get older, no matter how your mattress is made.