Helmets get dropped, crushed, slammed, and are put into many other situations that may compromise its structure. Helmet manufacturers suggest that you get a new helmet every two years so that if you should fall off the helmet will protect you like the day it was made.
This suggestion is also around to protect the helmet manufacturers from lawsuit in the event of a malfunction and so that they might sell more helmets.
If you have a quality helmet like a GPA, Charles Owens of International you shouldn't have to replace it nearly as often as you might say a Troxel Legacy helmet