Change your helmet for any of the following reasons:
If dropped helmet or had some other heavy impact.
If over 5 years old in British weather.
if over 3 years old in solid sunshine states.
If you are bored of the colour/style.
Caveats: I dropped my helpmet and IDK how old it is, but I still use it. I guess I should change it and this thread has reminded me that I should and for that, thanks! I see it a bit like tyres. If the bike has tyres which are over 5 years old, I'll still ride the bike but slowly. If I want to push the sound barrier then I'll ensure tyres are < 3 years old and in good condition. Then again, my tyres dont last longer than that anyway.
Anyway, the above is just the way I roll, not necessarily correct.