If a helmet is cracked, dented, or damaged, it needs to be retired. But regardless of visible damage, anytime your helmet receives an impact that leaves you thinking, "Ok, I would be badly injured if not for my helmet," then it should be replaced. The helmet's buckles, straps, and other components should also be free of any cracks, frays, or tears, and the internal foam should be secure inside the polycarbonate shell (if one). Any damage aside, most helmets should be replaced after 10 years, and to be safe, reduce that to 5 years if you climb regularly.