Every person is different and like others said, you must go test out some mattresses first before buying. If you buy a mattress online without every trying it, just ball up the money and throw it away instead. It would be the same thing.
I, like you, was in serious pain due to sleeping on an old, shitty mattress. Finally decided to upgrade and man was I pleasantly surprised at the difference. I feel like a million bucks now each morning. It's like night and day. I'm only 30, but before I got this new mattress, I felt like I was 60 years old getting up out of bed each morning. Aches, pains, poor balance. And yes, I eat right and exercise regularly, so it wasn't that. I'm only 155 lbs. Run, stretch and work out regularly, wear the same pant size that I did in High School. So I was like, wtf is wrong w/ me? Why Do I feel so shitty all the time.
Answer was the bed I was sleeping on. I probably could've spent more, but after a trip to a local Sleepys store and doing some research online, I decided a firm latex mattress was best for me, and I bought
this. No joke, I couldn't be happier. I've only had it for a few days, so can't tell you how it'll hold up over the years, but the difference in my body is amazing.