Helmets (and car baby seats) are affected by a lot of outside conditions. Does fiberglass breakdown? yes, but nobody knows when. Cars from the 60's are not falling apart. Do nylon and Dacron liners come apart from hair chemicals and sweat, yes but they all take different time frames, do plastics that are in the sun all day breakdown? yes, but in what time frame.
It usually comes down to the lowest common denominator, the plastic clip that straps it on your head, staying attached to your head is important.
4-5yrs old stored in a box at the store? probably hasn't changed at all, sitting in the window on display, that could be different.
Fwiw, my street helmets, which I plan on never testing are various ages, my sporting helmets which probably will be tested at some point are never over 5yrs. personal comfort level, I have no data to support my thoughts.