The only actual testing of longevity of which I'm aware was done by BHSI when they sent off a 10-year old well-used and sun/UV yellowed Bell Biker to be tested. It passed the same certification tests that same model had when new with no noticeable degradation due to age and wear/tear.

I see no reason why keeping your helmet stored under reasonable conditions for a few years would affect its performance. Of course, if you race, there'd be the slight risk that some new certification becomes mandatory in the future and renders older helmets unacceptable for competition use.