What I hate about this, is that every time it comes up, nobody ever really goes into details of why. What is no longer safe? What breaks down? Or is it updated standards? Just fucking get into the reasons so people can make rational choices, instead of just saying "trust me bro."
The general range for replacing one is after around three to five years of use
After that long, I'd be replacing it out of boredom and wanting something new. LOL
The question is, would having an old helmet still be better than no helmet? Because trying to convince someone who already resists wearing a helmet, that they will also need to replace it ever few years, might just turn them off forever.
Helmets off gas from the foam and it loses its ability to absorb impacts.
Also, if you are ever in an accident where your helmet takes a hit, get a new one. They are more like airbags than people realize, once it absorbed an impact, it's done.