There are 2 reasons for replacing a helmet (the way I see it).
First, if the pads are warning out, falling out, etc., or the strap has issues, or the rear retention system (most newer helmets have them) is faltering in some way... and the whole thing looks like it has seen better days... (note that if the helmet model isn't too old, you can sometimes find replacement pads for the inside).
The second reason is because, well, it's old. Before you think "hey I don't need the latest model and style" - remember that the styrofoam inside the helmet changes over time. When you first buy it, the material is actually softer... and it hardens a little more every year. If you leave it out in the sun for long periods of time (like women in their 40s on the Cote d'Azur) it will accelerate the aging process. The worst situation is to leave it in the back of your car while exposed to the sun.
When the styrofoam becomes brittle over the years, it will not protect you as well as a newer helmet. When you crash, the helmet is suppose to crack and dent and basically distribute the impact, ruining itself while your head stays in tact. A helmet that is baked with age will not crack in multiple places as easily and will have no "give" on impact.
I've had some collisions with cars when I was riding at 30mph plus (why can't they cut me off when I'm climbing at 6mph? :) and done acrobatics into windshields and over hoods on numerous occasions. I've also hit quite a few trees etc when I did a lot of mountain biking. I think I've gone through about 8 helmets in 15 years.. usually because it did it's job once and it was time for a new one. If you see any cracks on the inside of your current helmet and its been 2-3 years, its probably time for a new one. It is recommended a new helmet every 3 years or so for maximum safety even if you never crash it (though I think you could stretch this to 5 years if you don't ride that much and store your helmet in a dark cool place).
Actually I think there is a more technical term than styrofoam as I doubt that is actually what they call it these days.
Funny you ask this question... my new helmet arrived in the mail a few hours ago!
| current weight: 158.0