My old Bell Star is noisy as hell, floppy, and stinks. My new one fits perfectly, is quiet, and so fresh. A good helmet makes a HUGE difference in your day-to-day riding enjoyment.
i do quite a bit of work with helmet specialists in australia. they said snell's five year replacement recommendation was quite arbitrary as a decent liner is very stable unless exposed to solvents, chemicals or uv light a lot. ( e.g. never wash your entire helmet with detergent!).
however they did say that generally helmets keep improving so they think the five year suggestion is still handy simply because you'll probably have a better helmet... manufacturers are slowly incorporating safety features like dual or variable density liners and little changes to reduce rotational forces, going beyond the requirements of our sadly outdated international helmet standards.
re: impacts, they weren't overly concerned about a helmet just falling off a bike and hitting the ground... the pvc liner usually will only start to crush when it reaches a high level of G forces so personally they felt it was more when you had an accident and you hit the ground hard enough to at least feel briefly stunned.
for legal reasons i'm sure they wouldn't make these statements public but thought i'd just share them.
Did they mention anything about the foam gradually compressing over time? My old Bell Star has clearly gone up a size when compared to my new one. It's about eight years old. Fit is a big part of effective protection.
totally agree kuhika. these guys said an expensive helmet is next to useless if it's too big and can be pulled half off your head even when strapped firmly. i'll ask them about foam compressing but imagine the foam shouldn't do that... i'd wonder if that would be the cheek pads actually, and possibly the inner comfort liner too? e.g. the bit you can pull out to wash occasionally?
i know mobs like suomy sell different sized cheek pads for when they get compressed or if you want a slightly tighter or looser fit around the cheeks?