If it's still in good shape and you feel comfortable on it then there is no need to replace it yet. Especially so soon.
I'm 28 going on 29 and still sleeping on the same mattress my mom bought me when I was 15.
So the mattress is 13 going on 14 years old. I'll probably replace it in at least 5 more years though as I would like a bigger one as this one is currently just a twin mattress.
Other than that it's in good shape.
I'd say you should at least be good for a few more years if you want to get a new mattress. Otherwise you might not have to replace it for at least 14 more years.
20 years is certainly long enough for a mattress and as far I would go unless I couldn't afford to buy a new mattress.
After all an old mattress beats sleeping on the floor.