The foam part is made for one time use, and after crushing once it is no longer as protective as it was, even if it still looks intact. Plastic shells can hide the foam damage. A few helmets made of EPP foam do recover. If in doubt, contact the manufacturer for an inspection. If your helmet is more than 10 years old or has a cloth cover, we recommend that you replace it.
Many manufacturers recommend replacement every five years, but some of that is just marketing.
Deterioration depends on usage, care, and abuse. If you ride thousands of miles every year, five years may be right. So do over 50 cities and counties. We have the current list up on our Web server. About a third of the population of the U.S. lives in one of those states or cities.