Comfort is subjective. Around the world, people sleep comfortably on straw mats or the floor, in hammocks, and on mattresses of every description. This is one category, at least, where what feels good is good. If your mattress has developed peaks and valleys or sags in the center, or if a coil is poking out, or if it's just not comfortable, you need a new one. Mattresses wear out at different rates.
Many people find an inner-spring mattress atop a box spring most comfortable.
Others like a foam mattress on a platform. Still others sleep well on a futon. Some people like water beds. Whatever works for you is okay. If you share your bed, take your partner along for the test.