A standard engineering factor of safety is 2x. Get lawyers involved and you get 3x. Get the marketing guys involved and it goes to 4x. So as long as you do not damage your helmet from crashes or dropping it severely, you can take that 5 yr life and double it. I buy $500+ helmets, there is no way I am retiring them in 2-3 years.
10 years is a more reasonable life for the helmet. I also do not leave the helmet locked on my bike in the sun. Easy enough to take it with me into work or where ever I may be. Plus that way it cannot be stolen.