It’s about 75% dust mites by now. They’re very supportive little buggers! But at 15 years you should replace it and put an encasement or waterproof cover on the new one. And a mattress pad on top of that. Then you’ll be able to wash the absorbent mattress pad while your mattress stays pristine. An encasement has the advantage of blocking out any bedbugs you might bring home. You don’t have to go expensive, I recently got an Ortho Signature Club II for about $500 including delivery and disposal of my old one. If you can handle getting rid of the old one by yourself, Bear makes very good arrives-in-a-box mattresses. No fiberglass, meets Canadian standards.