I don't doubt the industry errs on the side of replacing sooner rather than later for several reasons. Obviously, it is to their advantage to encourage the sale of more helmets, but they have skin in the game as far as liability too. They have no way of knowing how a helmet has been used. I'm sure a lot has to do with the conditions the helmet is stored in.
Big difference between bringing your helmet inside your house after a ride and letting it sit in a sweltering shed, car or garage during the summer, or the reverse in freezing cold in the winter.
Some common sense needs to be applied. My wife has a helmet that is now 5 years old. The first 3 years it essentially went unused, but was stored inside the house. The last two years, 1,300 miles and about 124 hours of riding, still kept inside when not being used. And still light use. I believe safe to use.
My primary helmet is 2.5 years old, with about 800+ hours of riding on it. No crashes. I'll probably replace it in about a year. Though more to get a style change and get MIPS. What I have has Wave Cell and it's fine except you can't slide glasses into. And it will serve as a backup helmet or a loaner.
I went looking for some info on helmet replacement, and found this article. Here's a a key quote from the article:
"That recommendation is largely based on research conducted by engineering firm MEA Forensic, which found in tests of hundreds of helmets that age did not significantly affect the performance of the protective foam in helmets up to 26 years old."
When You Really Need to Replace Your Bike Helmet- Consumer Reports
As others have mentioned, other parts of the helmet may wear and there can be improved features. So, isn't just about age. But it does seem that a lightly used helmet, stored inside, could be as good as it was when new for 10, 20 or more years.
Looking at the Virginia Tech helmet testing, I see they tested the Giant Relay MIPS helmet and it has a 5 star rating (all the stars) and manufacturer's list price is $50. So, price is really no excuse. For the price of two Starbucks coffee drinks a year for 5 years, you could have a new helmet. Certainly not so expensive it is worth taking a risk.
5 years ago when I got seriously into cycling, I showed up at a local bike shop with my 20+ year old, dad-bike helmet and the guy wasn't going to let me leave without buying a new helmet. Or rather was going to do everything he could to get me to buy a new helmet. Which I did, Something like $80 with MIPS. No regrets and that helmet did it's job in a serious accident involving a car. Helmet cracked, my head not a scratch. Other parts of my body weren't so lucky.
I don't care if I might be getting duped by "big helmet" to buy a new helmet every 3-5 years. I typically spend more on tires in one year than a higher end helmet costs.