I'm looking for a new helmet and obviously last year's models are cheaper ..or older. I see a lot of 2010 and even 2009 models for good prices. So they say replace your helmet after 3-5 years. Some people say that clock starts when you start wearing a helmet and sweating in it and exposing to weather etc. But I've seen some others saying the foam starts degrading immediately. So is it worth saving a few bucks to get a new 4 year old helmet?