So, apparently this is a thing, some confirm it, while others deny it, still couldn't find any solid evidence in this regards, though there is this quote from another forum that I found convincing:

Hello everyone. This is "Skippy" (a.k.a Robert Miller) from The Service Pavilion as the guy from Alaska called me.

Hello to you as well Mackdaddy. Thank you for the kind words about our service to you.

Ok let's clear a few things up. Many things people say are taken out of context, everyone needs to remember that. Examples are given to people and then they seem to be taken as law or concrete facts, when is was just an e-x-a-m-p-l-e....

Mackdaddy called and asked some questions. One was helmet life. Arai does have a suggested life on a helmet. What other companies say or suggest? Is up to them. Arai's suggested life on a helmet is 5 years from first use. (After it has been on your head, and that even means just trying it on!) Or seven years from the date of manufacturing. The warranty is based from the same numbers. 5 years from first use, 7 years from date of manufacturing which ever comes first.

This is not about selling more Arai helmets. This is about the Arai product doing the job it was designed to do. Let me elaborate. (Mackdaddy, this story will sound very familiar to you...as well as anyone else that has spoken with me about this) This is from years and years ago at Arai. 2 new Arai's were taken off the production line. One helmet was worn for 15 minutes. One was never touched. They were put back on the shelf. A year later they were both analyzed. They both showed some change from when they were born. The one worn for 15 minutes showed greater changes than the one that was not touched. Now, pro-rate that to a point that the helmet shows changes that are unacceptable to what Arai builds their helmet standards to...5 years. That was on a helmet that was worn for only 15 minutes!

Now what about the rider that wears it maybe 5-6 times a year. Then the rider that rides all summer on weekends 25-30 times. Then the rider that rides every other day. Then the rider that rides daily. Then the rider that sweats like Niagra Falls. Then the rider that barely sweats. The rider that eats a lot garlic in his foods. The rider that uses lots of hair products. The rider with no hair. The rider that has a little hair. The rider that smokes. The rider that lives in a rural area. The rider that lives in a mjor metropolitan city. The rider that rides damp weather more then not. A rider that lives in Texas versus, Florida versus, NYC versus, Seattle versus, Las Vegas....etc.

Each persons pH balance (acidity in your sweat) factors into helmet life. Atmosphere of where you live factors in. How you store your helmet. How you care for you helmet. The oils in our skin and hair. (Like P B G commented on) Shampoos we use to clean our hair. Conditioners. Gel. The stuff in our hair for those of us who don't wash our hair. The list goes on and on. Helmets are a consumable item. Like tires, batteries, fuels, oil etc. Do tires not dry rot sitting around? Does gas not loose it potency over time? Do batteries not loose a charge not being used? Does untreated aluminum not instantly oxidize? Do materials used to produce a helmet get effected by the same things? Yes.

In regards to GSwannabees comment on styrofoam. There are many different qualities of "styrofoam" out there. So the comment about styrofoam is very durable is really not that accurate. Some are very cheap and break apart easily. Some are nearly indestructable. There are literally hundreds if not thousands of different styrofoams out there. Within the helmet production world alone there is over 35 different types of styrofoam, expanded poly-styrene, etc. There are so many variations and formulas to create it as well. The quality control of how itis produced and shaped is another factor.

Here is an example I share with many of my customers. I sold a helmet to a guy that lives in Seattle and he rides daily doesn't even own a car. Fit and sold him a helmet at the Cycle World International Motorcycle show stop there in December one year. A year later I saw him at the show again. He brought his helmet to me and said this thing does not fit like it used to. I took it from and him and could not believe what I saw. The interior was almost black (Blue is the color when they are new and clean) the helmet on the outisde was almost spotless. The cheek pads were shot and loose. The headliner was completely shot as well. The EPS (styrofoam as some call it) liner showed movement inside the shell. And with a new Arai there is NO movement. (When the EPS liner does not conform to the interior of the shell that is not desireable with an Arai prodcut. Because they are not that way from new, so something has changed) The EPS liner had shrunk. Which would fail a crash inspection from Arai if the liner moves or has space between the shell and liner.

I asked him how many miles did he put on the helmet since a year ago. He thought about it and said about 34k + miles....I asked him how many oil changes? 6 he said. How many sets of tires did he go through? 4 fronts and 6 rears. There was nothing that could be done to make his helmet fit better. I told him and explained to him the same thing I just told all of you here. His helmet was not good to use. He was unhappy with me when I told him he should replace it. He was upset because he spent $600+ on it with me only a year ago. How could this expensive helmet do that? So I explained it to him again. So after he understood these points and thought about it over lunch and few hours later he came back and ordered 2 helmets. He expected to be riding more this coming year than he did before!

I have had more people than I can remember now that encounter the same thing. They ride so much they wear out a helmet in 1-3 years. I see them on a regualr basis at events to look over their helmet and help them monitor the helmet life. They have seen and understand this first hand. Now myself? I sweat a lot, I ride street, off-raod, race some local flat tracks, ride pitbikes. My helmets last me for years. But I see them change. I feel the fit change as well. And the change is more then just putting a new set of cheek pads and a new comfort liner can do. They get replaced before I ever get to the 5 year mark. I really don't even think I ride that much either.

Now I see helmets that guys are wearing that are 9,10, 15 years old. They look great on the outside. But the interiors are shot. Every person is different on how the helmet lasts. But the rule of thumb Arai has set is for the majority of riders that have a Arai helmet.

Here is the fact. You can wear out a helmet from just using it. No different then a tire, chain or motor oil. All products we use have a life time. There is no set milage or time for a helmet. Arai suggests 5 years based on what is expected of helmet to do when/if it is ever needed.

Now Mackdaddy, there is no theory about this. It is fact that helmets wear out. I never said to you on the phone after 15k miles or more you should replace it. I gav you an example of what people experience. The time isn't an accumulative time of riding you have done, it is time from first use.

Every helmet sold (in the U.S. anyway), must have a production date by law somehwere in the helmet. It is true, every brand does have a sticker, tag or a marking of somekind. Arai now etches the date on the chin strap d-ring. (pretty cool)

I read that someones post asked about helmets that are in shops, sometimes have old production dates. Let me address that from the Arai production standpoint. This is where the 7 years from production date comes into play. An Arai helmet can take up to 2 - 2.5 years till it can reach the market (dealer showroom floor), which is acceptable from the manufacturers standoint. Like P B G stated it will always be first in first out with product sold. That is practiced worldwide with anything. Ok maybe except for some wine and cheese! If helmets are not sold by a Dealer, they do not order more from the Distributor. If the Distributor does not sell to the the Dealer, then they do not re-order more from the Manufacturer. So Arai does not build helmets and just set them on a shelf. Every Arai is made from an order. If old helmets are out there, it is a Dealer (like myself) not doing of good job selling their inventory to keep fresh ones moving in. Or a Distributor over ordered an item and it is still being sold to dealers to try and move it. That is usually why we see old helmets on shelves around the country.

So I hope this cleared some questions up and maybe made things clearer on my standpoint. I do not consider myself a salesperson. Never have and never will. I am a kid that has been around bikes all 36 years of my life. I am just a kid named Robert Miller. I chose to focus my service, knowledge and experience on the Arai helmet product line. I used to race for a living. I used to work for Arai Helmets. Now I have The Service Pavilion featuring Arai Helmets. (we just turned 7 years old!) I am not a salesperson. I have products and information. If you buy something from me it's cool. If you don't buy something from me it's still cool, because you probably asked us some questions you had and got some answers. Maybe learned something too and got a direction for you needs. But that is all cool to me. I do want to sell helmets to people. It is needed to survive you know? But it is about information. Have fun riding everyone and be safe out there.
What do you guys think? Personally I wouldn't trust my life on it, so I think I would replace my helmet every once and a while (what a nice excuse I've got now since I like helmets!). And another thing, would this also apply to all safety gear?

Edit: here's the link for the thread that I took the quote from, in case someone was interested in reading more opinions.