When I could no longer find replacement cushions for our '06 Shoei x-11's two years ago I sent a note to Shoei asking for some cushions. They told me in no uncertain terms that they didn't stock parts for helmets that old and my helmets were 6 years past their reliability why hadn't I replaced them every 5 years. I asked for test results that demonstrated a significant difference in break/crash/drop test results of older "worn" helmets, and I never got a reply. It may be true, but I've not seen any test results to show that a 10 year old used but otherwise undamaged helmet fails (or passes) the DOT or SNELL or ECE etc., tests.
If there is no deformation or damage, if the 8 year old helmet still fits as snug with new cushions as it did when new, is there a need to replace it? And saying "Do you want to bet your brain matter on it?" isn't the answer. Sometimes I'm an annoying skeptic... Why is 5 years the magic answer? Why not 6? or 4.5? or 7? Ask anyone, the standard answer always seems to be 5 years. I've even read articles that state you should watch how long your new helmet sat on the shelf before you bought it. Does that mean the 2 year old NOS helmet you just bought on-line has only 3 years to go?
Given worney's info about foam possibly hardening or disintegrating over time, I would agree that would be something significant, but over what time frame? And how do you measure it? Are there any test results?
I like Shoei, they fit our heads, and wear them all day with no hot spots. Two years ago I found 2 Shoei QWEST on a Black Friday-type sale for $225USD.