A.V.
Any mattress can "go bad" and be less supportive. If the mattress is bowing and leaving gaps, is it not safe. I'd replace it. You can use that mattress for a toddler bed later and get more use out of it, but it doesn't sound safe as it is. If it is JUST that the sheets are short, get new ones, but if it's really the mattress, it's not something I would keep.