As the plasticizer leaves plastics, it becomes embrittled. You want the EPS to crush smoothly -- if it cracks and displaces, it will not be in place to absorb

As the plasticizer leaves plastics, it becomes embrittled. You want the EPS to crush smoothly -- if it cracks and displaces, it will not be in place to absorb energy and cushion the impact further. We replace our bike helmets every ~6 yr. Source: Have listened to the crackle of decade-old plastic kayaking helmets under tensile/compressive stress and watched them shatter like glass after impact with a concrete floor. New helmets from the same manufacturer did neither.