Not every 8 years. Clever marketing but that's total BS, especially if you have and use a mattress pad. The stuff they claim gets in a mattress would have to go through sheets and a thick pad. That's just a way to convince people to buy much more frequently than they should. Many people keep mattress 15-20 years or more depending on the original quality and how we you take care of it.
Get a new one only if you want one, NOT because of what someone in the business of selling mattresses tells you. If it is sagging or if you want a different model or style, or is it is stained and just worn out, then consider a purchase.
Let your own comfort be the judge. 4951