A few comments:
I bought a 9 year old helmet to Shoe's US headquarters near my home for inspection. At the time they told me that they will pass helmets that are older than five years provided that other issues are not present. I wish I could remember the exact lifetime - 8 or 9 years, I think. Beyond that lifetime, they won't pass the helmet no matter what, but it is longer than 5 years. My 9 year old helmet had significant compression spots in the EPS liner anyway, so the exact acceptable lifetime didn't come into play.
(Best guess: I used the built-in Honda helmet lock, which hung the helmet above the muffler and probably too close to it. Heat rising from the muffler softened and eventually compressed the liner.)
WASHING THE PADS:
If you have removable pads, wash them reasonably often. According to Shoei and Arai people that I have met, your perspiration, natural skin oils, unnoticed residues from shampoos, conditioners, colognes, and other personal care products do cumulative damage to the energy absorbing liners in your helmet.
RINSE YOUR HAIR VERY WELL
When I shower, I take extra care to thoroughly rinse the shampoo from my hair, and then rinse some more. In addition to giving life to my helment, it has also helped me avoid "itchy scalp" on very long rides.
There are many fine helmet liners on the market, but LDComfort released their own version last June, after testing prototypes in the last Iron Butt Rally and on other very long distance rides. They aren't pretty, but they are popular with many long distance riders I know. I have one and like it.