For me personally, it would depend on the condition of it. Our cot mattress has gotten loads of use and wear as we have also used it as a travel mattress, so we bring it with us camping, and it's been chucked on dusty floors in holiday cottages and grandparents' houses for 3 years now. Not to mention, it's faced some nappy leaks and vomiting bugs and even with a cover, wouldn't be something I'd be comfortable putting a new baby on. Same for our cot bed mattress which has seen now almost 3 years of use. I would feel more comfortable getting new ones. But if the condition is good, they haven't been stored anywhere damp or musty, and structurally they still offer lots of support, I'd be more comfortable reusing them, but I think it just really would depend on what shape they're in. I think the concern is usually do to sagging, if they sag in the middle and baby could get stuck in the dip, or respiratory irritants, like moulds, being harboured in the fibres.