My mattress is at least 20 years old and is collapsing in the center. The matching mattress (this is half of a twin set) was replaced years ago.
I'll flip my old mattress over and see if that does any good for now. If not I'll ditch the mattress and just sleep on the box spring. For four years at college I slept on a sheet of plywood with a 2" thick foam rubber pad on it and I was just fine.