In reply to fire_munki:
I think it's very hard to really know and maybe I've been massively pushing my luck, but I've happily (stupidly?!) been using helmets regularly up to a decade or a bit more. I've got an Elios, which is a hybrid, that's at least 10 years old (I'm wearing it in photos taken in winter 05!) and it still looks fine. It's actually currently being used by a friend's daughter in Scotland. When my mate said was it OK to lend to her (he had for various complicated international house moving reasons!) I said absolutely, but when its someone else's kid you suddenly become more worried about it than when its (literally) on your own head! So I suggested that if she gets more into climbing, she considers getting her own helmet sooner rather than later because of the age.
A couple of my oldest helmets now reside in the dressing up box that my parents keep - for when all the various grandkids come to visit. Hence my early 90s Snowdon Moulding carbon fibre lid in disco ball glittery purple has been reimagined as a coppers helmet, a soldiers helmet and part of a suit of armour at various time! But it actually still LOOKS fine for climbing in.