What do you do with your old helmets?

It is said that MC helmets should be replaced every 5-7 years. I have 3 perfectly good helmets that are older than this, so they don't get used anymore. One still looks brand new. There seems to be recycling for everything these days, what about old helmets? Seems like such a waste to throw away an otherwise good helmet...
I didn't realize they were supposed to be replaced if they'd never been involved in an accident. When I quit riding about 10 years ago I was still using a helmet I bought in the '80's, There may still be some around here that I bought in the '70's. Most people here in KY no longer wear helmets. If I were still riding I'd still be wearing a helmet. I may be hard headed but I don't think it compares to asphalt.
Lots of debate about the 5 year deal. The reason given is the materials used degrade over time and use and even though a helmet looks new it *might* not perform as designed in an accident. How would you know? Is it an attempt by the makers to sell more helmets or are they really designed with a useful life of 5 years? I've seen helmets at thrift stores, better than throwing it out. If I were to sell a used helmet I'd mention to the buyer when it was made and let them decide if that's important or not.

I replace my bicycle helmet every 3-4 years. Good ones aren't cheap but it's the only head I have.
Hedgesville, WV
I guess you just have to throw it in the trash. The recycling rules have been getting tighter and most things that we use to recycle are not accepted anymore. As far as expiration goes, unless you store it outside in the UV or subject it too some crazy storage conditions I go by the condition of the liner foam. If that is getting brittle and crumbles or packs down and does not spring back then its time to replace it.
Seattle-ish, WA
True helmet story:

About 20 years ago I had to do medical research project for a requirement. I started doing one on motorcycle helmets, safety aspects etc. (mainly because I hate motorcycle helmet laws myself). Anyway, I ran across a medical report from a British doctor in India before WWII. He pointed out that for some Sikhs, religious practice and customs with regards to headwear precluded helmet use. For them, he recommended, and I quote exactly, "... a very well-wrapped turban."

Yeah, that's gonna help...

I thought maybe I could convert, then claim a religious exemption from the state of Washington. Since I lived in Seattle at the time, it probably would have worked.
I have a Fulmer 3/4 helmet I wear in the winter and it’s at least 10 years old. Liner and padding are still in great condition. I should throw my 30 year old Bell and HJC helmets that I no longer wear in the garbage, but they’re still on a shelf in the garage.
If you ride an RC45 there is only one helmet you need to wear... a
John Kocinski replica that celebrates his 1997 WSBK championship...
Since its an Arai new lining is available from the manufacture... I
keep it all fresh and clean by giving it a bath in warm water and
lite soap...

Me and John Kocinski Laguna Seca




Side story...
I rented a bike in Hawaii (no helmet law) long ago, and they acted shocked I also expected a helmet.
I had them reach up and get the one with scars already on it...
It's probably a combination of factors that led to the "rule" of replacing them every few years. Some would say if you've ever accidentally dropped your lid, you should replace it as it is only designed for a one-time impact and microfractures are likely. It would be compromised in the event of a crash.

There's the economic factor obviously, but more importantly technology marches on and better materials become available that result in safer, lighter, more comfortable lids. Most people don't bother to wash the lid liners or cheek pads, and those absorb sweat over time making them a possible petri dish. Or some folks overwash them until the liner starts falling apart.

My current (3rd overall) helmet replaced a Bell lid I'd have for over 5 years. This new one (HJC i70) has some nicer features and it's always nice to upgrade.