I double checked this a few years ago, when we were painting skidlids.
Most manufactuers recommend a shelf life of 3-5 years on a poly prop or thermoplastic helmet - IF its kept in a cool dark enviroment. So always buy a lid thats been stored in its box not the one off the shelf.
The manufacturers of composite lids, i.e carbon fibre, fibreglass etc. said their lids have a shelf life of at least 5 years if stored in a cool dark enviroment.........
All helmets sold within the EU must have a manufactuers date tag stitched inside.
A lid should be replace if it has had a 'significant' impact - their words. Ask to define 'significant' drew the response 'Er.......'! Basically, dropping a lid onto a flat hard surface from hand height won't harm it, other than chipping the smooth finish. Dropping it from the same height on to a sharp edge or point is another matter. The best advice I got was 'use your common sense'; if a lid has been used to save your head from impact during an off, then its time to replace it! If someone accidently knocks it off a table in a bar and marks it, its probably okay to wear (and don't blame them either, you should have put it somewhere safer anyway!!).
I've yet to hear of a case were a skid lid is a main factor in an accident which ends in a claim - however the way things are going its only going to be a matter of time..... Esp with some folks insisting on wearing lids that don't have the relevant test specification markings - something to think on the next time your mate decides to wear that repro viking helmet for the charity ride out........