Do I really need to replace my helmet every 2-3 years if it hasn't been in a crash?
The Bicycle Safety Institute disagrees with the 2-3 year rule. They have a good page on replacing helmets.

I usually replace one when the foam rubber gets old and crumbly, when the adjustments quit working, or when I damage it. UV can deteriorate the plastic if you leave it outside a lot. And, if you ever "use" a helmet, i.e., crash and save your skull with it, you should replace it because the styrofoam and outside plastic can weaken.