How long does an unused helmet keep for?
There is a common "fact" out there that you should replace a helmet after 5 years, regardless of use or crashes as there is concern that the crush properties of the foam may have changed (e.g., hardening) which could provide less protection.

However the data doesn't seem to bare this out.  A recent study did extensive testing of hundreds of used helmets  collected from the general public showed that there was no relevant impact performance change with age.  The helmets tested had been used but showed no signs of damage and were manufactured between 1987 and 2013.  

These results show that if your helmet is not damaged there is no need to replace it simply because of aging.  Now likely at some point there is an upper limit whereafter a given age it will need to be replaced, however that limit appears to be well beyond  5 years (some of the tested helmets were over 25 years old - and they still passed crash testing).  The most important thing appears that they have been certified under a recent standard and the helmet is not recalled.