i get nervous after two years.
there are a couple of helmets that are older, and only come out on special occasions, but i'm also anal about cleaning them after each ride.
unless you live in a very mild, fairly consistent climate, things like the heater and A/C in your house doesn't help them any.
styrofoam doesn't break down easily, but it does lose it's ability to absorb impact over time because some of the tiny air bubbles dissipate.
(wanna see something weird? go diving and bring a styrofoam cup with you. they compress as you go deeper.)
(not my hand, not my cup).