I have a brand new medium-firm XL twin mattress I just bought that is not firm enough for my bad back.
It is in brand new condition, slept on maybe 5 days and was protected by mattress cover from day 1.
It is 38" x 80" x 12" high and has hundreds of coils and many layers of cushioning on top. It gets a 4.5 star rating on Amazon. It would be perfect for someone who does not have back and hip problems as those reviewing it rave about how comfortable it is.