Originally Posted by linberl

Snell says every 5 years. At $40-50 I consider it a lot cheaper than my medical co-pays in case they are right. To each their own. As for Bay Area weather, if you actually live here then you know we have micro-climates. And some are quite warm compared to others; in any case, how one rides and the effort expended will also impact how much your body heats up and the need for ventilation as well as your own personal physiology.
From Snell:
Why should you replace your helmet every five years?

The five-year replacement recommendation is based on a consensus by both helmet manufacturers and the Snell Foundation. Glues, resins and other materials used in helmet production can affect liner materials. Hair oils, body fluids and cosmetics, as well as normal "wear and tear" all contribute to helmet degradation. Petroleum based products present in cleaners, paints, fuels and other commonly encountered materials may also degrade materials used in many helmets possibly degrading performance. Additionally, experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five-year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five-year helmet replacement is a judgment call stemming from a prudent safety philosophy.