Feldman

If your helmet is more than ten years old, whether or not it has deteriorated, try a few new ones on--fit and ventilation have improved substantially. Specialized and Giro, to name just two brands, have abandoned the old fitting system of patching pads in and have adjustable bands for fine tuning fit. Also ventilation continues to improve. I alternate between a midrange Giro and low/midrange Specialized--hot summer days, my head is cooler than on the helmet I would have been wearing 20 years ago.

Myosmith

Here are the reasons for which I will replace a helmet:

• Damage from a significant impact
• Failure of a component such as the adjustment mechanism
• Significant improvement in technology affecting comfort and/or safety
• Found a helmet that better suits my personal preference

I have a couple of older helmets around that I keep as back-ups or that I keep in my vehicle for unplanned riding opportunities such as test rides.

I have never replaced a helmet based on an arbitrary schedule.

Junkbeast

Children's car seats have an "expiration date" due to what they say is degradation of plastic or something and it becomes unable to protect fully. That's all regulated whether it's good science or not. There's no such program for helmets that I'm aware of but it could be true still. Your mileage may vary. Do what your gut says.

jefnvk

I bought a new Bell helmet on closeout after four years. Still haven't worn it, just waiting for my last one to fall apart or get left somewhere. It was far more a cosmetic and hygenic/cleanliness decision than a safety one tho, its ready to go when my old Wally World Schwinn helmet becomes too gross for me!