Polystyrene doesn't wear our or degrade from age. It does lose it's ability to cushion and protect with crashes and even hard bumps from just throwing around.

If you have an old helment, consider replacing them. You can find real bargins if you shop around on older models. Newer helmets are lighter, have been straps for fit and adjustment, and more holes for cooling. Also you can find a new helmet for close to the price of the kits with replacment pads.