Helmet Age vs. Safety: How Old is Too Old and Needs Be Retired?

TL;DR, 3-5 years mostly based on how much/how big I crash in the helmet, and I store it indoors in a dark cabinet so it doesn't degrade. 

I think the official line is that helmets last 3-5 years, but it's meant to be 3-5 years of active use. If you are storing your helmet in a dark, cool, and dry place it can probably last for longer as the material isn't degrading as much. That being said, I usually replace my helmets at the 3-5 year mark of ownership anyways since I generally have crashed in them at that point and however 'light' the crash feels it's probably decreasing the safety and capability of the helmet. My exception has been for full face helmets since I don't ride with them as much so I have held on to those for quite a bit longer or until I crash.

I wouldn't mind buying a NOS helmet from a warehouse if it was 3 years old since its probably still good but I wouldn't go for something over 5 years old as by the time I have it for 3 years it's already 8 years since being manufactured. That's quite a bit of time, plus newer helmets are adding safety features and getting better for the most part. I had a brand rep tell me that the big contributor to helmet degradation is UV light and to a lesser extent temperature swings and moisture, hence the advice to keep helmets in a dark, cool, and dry place. That's pretty similar advice to tires; leave a bike outside long enough and the tires start to dry out, crack, and become quite wooden just from environmental exposure. 

As an extreme example, I used to work in a shop where someone came in with their 15+ year old helmet (I think they stored it in a garage/shed and it was subject to Canadian winter + humid summer). It was in perfect condition other than being old. There was another MTB helmet we had in the shop from someone who had crashed and it was visibly cracked but it was only 2-3 years old (and stored inside the house). Someone stood on both helmets and the old helmet disintegrated while the new, cracked helmet held it's shape. You could pretty much break the old helmet apart with your hands, but the cracked helmet survived a few blows with a mallet before it finally split in two. Obviously this is an N=1 example but it backs up what I've been told by helmet manufacturers. 

I just get a new helmet every 3 years or after a crash. Helmets aren’t that expensive in comparison to bikes and there’s just no point in taking a risk when it comes to your brain. 
 

I’ve had a heavy concussion with partial amnesia once and it’s not something I ever want to go through again so it’s worth the 50-60 bucks a year to know you are on top of your safety gear. 

There are a few highly rated (https://www.helmet.beam.vt.edu/bicycle-helmet-ratings.html) helmets that are $60-100. That's what I go for, makes it easier to replace after even a mild crash than a $300 helmet. And for open face helmets I've never really noticed much difference in terms of fit and function between expensive and reasonably priced ones. Think of helmets as a wear item, 3-4 years or after a mild crash.