The 3-5 year rule is because of UV rays breaking down the foam and glue. That's the reasoning, anyway. Same in the motorcycle world. Those helmet manufacturers won't warranty a helmet past 5 years and stop selling replacement parts (padding, visors) for the old models. My current helmet was bought 5 years ago. I ordered a new Giro a few days ago simply because it was 50% off and I was ready for something new.