There are different toilet seat materials, with the most popular being wood, plastic (often polypropylene) and ceramic. You can also find toilet seats made of soft padding with vinyl layered over the top. The choice comes down to comfort, medical needs, durability, and aesthetics.
Your toilet seat needs replacing only if it's broken, peeling, wobbly, stained or generally "past it" in your opinion. As long as you regularly clean your toilet seat and it's functioning well and serves the needs of your family, it's okay.