We have discussed that very question before (see the previous thread here). Manufacturer recommendations range from 3-4 years to 8 years (MET, cited in the linked older thread by goldfinch). At the time, I asked a materials science prof about the longevity of EPS and helmets, in particular. He said EPS does not degrade over time. Given that, other considerations factor in: EPS may not degrade, but the padding and straps may look ratty, you may prefer a different style, etc.