We have many customers come in and ask a similar question "Is my helmet still good?". This is one of those questions of the philosophical nature, the one that you probably already know the answer too, much like "should I take a shower today?" or "am I really in love with that person?". No, your helmet is probably not still in proper working order. This is kinda a big thing since proper "working" order means still having that life saving ability.
How can you really tell? Me and my co-workers favorite test... The hand break test!! If you can break a helmet with your hands... get a new one!!!
Picture: Spanish snapping a helmet in half with her mighty do-good arms
So what is a helmets life-span? Most manufacturers recommend replacement after 3 to 5 years depending on your climate. If your on a coast next to a sea think 3 years max, here in MPLS we usually get around 4 years sometimes 5 if your lucky and store it somewhere proper. There are also many things that reduce the life expectancy of a helmet. Some of those being dropping it, crashes (you should replace after even a single crash), puppy dogs eating it (this happens a ton!) and outside storage.
Picture: This is a helmet being locked to a bike, if you do this on a regular basis you should start thinking of replacing the helmet yearly. The sun is a helmet killer!
Why do helmets have such a short lifespan? They are Styrofoam, not a very environmental material but the best to make helmets out of. Styrofoam breaks down and gets brittle at a very quick rate (remember those Styrofoam coolers that would break after a month or three?). The reason they use it is because of the nature of the material. A bike helmet sacrifices itself to save your head. It is meant to crack and compress when your head hits the ground in order to slow the impact down. This also means that once that helmet gets squished and broken it is no longer good. This can also happen by simply dropping it (usually more than once at the same spot or from a high height).