Ah! Helmet longevity and replacement criteria. You'll get as many opinions on this as you will on which oil to use. Most just go with what they are told. 5 years and it's done. Most believe that a Snell rated helmet is better than a DOT only helmet. When the science, and real life accidents, have been showing us for years that this is not true. The power of marketing (or propaganda, as some will call it). Never underestimate it.
Understandably, most do not take the time to examine the data and come to their own conclusions. This is true in most every aspect of our lives. If you go with the 5 years and done, you'll probably be fine, though your wallet may suffer unnecessarily, depending on what helmet you have.
I know, 'What is your head worth'. And I understand the argument. So, why not purchase a new helmet every year? Or, every 6 months then? You see, we all draw the line somewhere. And as with every curve, some are wasting a lot of money, and some are living beyond the edge. Again, it's life. It will always be so.
To look at this question rationally, you first need to look at the quality of materials, design and construction. Have a cheap helmet? 5 years may be too long. Have a high quality helmet? It will, with care, do a great job of protecting your head for much longer than the 5 year recommendation. The part that deteriorates, (interior liner-padding) can be replaced on most mid-range, and all high end helmets. The inner and outer shell on a high quality helmet can give original design service for at least 10 years.
I have been dealing in Arai helmets since 2000. A very good helmet. They have a date of manufacture stamp in the D-Ring. Arai says '5 years from the date put into service, or 7 years from date of manufacture, whichever comes first'. Arai acknowledges that very little, if any, deterioration occurs until you start using the helmet. It is the oils, salts, and dirt that you grind into the liner materials that deteriorates them. Though most of the shell curing (hardening & strengthening) process happens within a few days of manufacture. The shell actually continues to get slightly stronger for quite some time after manufacture.
The other factor you need to take into consideration are advances in materials, construction and design which do a better job of protecting your head in an accident. Sometimes there are leaps and bounds in a 5 year period. Sometimes, not so much. Frankly, science is coming up against a wall in this regard. They are getting helmets to the point that it is very difficult to make any significant safety improvements. This is a good thing!
But the sad part about all of this is. As much as people want to adhere to the service life recommendations, purchase a quality helmet, and talk about 'What is your head worth'. 70% of non-professional riders out there are wearing a helmet at least 1 size, and many times 2 sizes, too big for them. The best, brand new helmet in the world will not protect your head if it is too big. Because a too large helmet is very likely to come off your head in an accident. Rendering the rest of this discussion mute.
