What is it based on? I understand they do, but what defines it as unusable? Crack? Becoming brittle? I have an Ecrin Roc that’s over 10 years old that’s still in good shape. I’ve replaced the innards once. Just like saddles have a lifespan. I’ve worn out four and am getting ready to retire the one I’m on now. All depends on how much use it sees.Helmets have a limited life span.