I just replaced my Bell helmet after about 3-1/2 years. However, my reasoning really had nothing to do with safety, but rather two other factors:

1 - that old helmet stinks to high heaven and no matter what I do, it still stinks.

2 - the old helmet's chin straps, although I've cleaned them, must be harboring some horrible pimple germs because they are making my jawline and under my chin break out.

Now that I've replaced the thing, I'm glad to read that maybe I should've been replacing it for safety's sake as well!