I tried a Sleep Number bed at a hotel. Hated the thing. Back when I was in university, I bought a simple foam mattress (4" foam) which was the cheapest I could find. I slept on it for a few years, then it became my guest bed. Every single person who ever slept on it said it was the best sleep they ever had!
If you’re only starting to get aches now, are you sure it’s not the way you’re sleeping or what you’re doing during the day? I can get into habits of sitting wrong or sleeping with my shoulders hunched up or something and I’ll wake up aching - it sometimes takes me a while to figure out what I’m doing that is causing the ouch.
I’ve been sleeping on a traditional futon on the floor for years. When I visit relatives, who all seem to have medium to soft mattresses, my back aches the next morning. They’re too soft for me. My wife wanted a bed when she moved in, so we got one, but remembering my sleeping problems in the US, I insisted on a platform bed that you can use a futon on. Maybe you need something firmer than a regular mattress.