2005 Performer Toscana, 2000 Kona Hoo-Ha, RANS Rocket
Contrary to what the LBS and helmet manufacturers may recommend, I read somewhere that helmets do not need to be replaced every five years*. The materials don't degrade or stiffen with age, or at least not enough to matter.
*Applies to helmets that have not been crashed and styrofoam helmets with a hard outer shell. Foam helmets with a fabric only covering (such as those made in the '80's) should be tossed out.
I replace my helmets when I get so tired of them that I'm willing to spend the $$$ for a new one. I'm cheap, so that only happens every 5-8 years. Then I keep the old one as a spare. I've got lots of spares! My newest one is only 2 years old.