2 types of lid, polycarbonate, which do go off withuv, and should be replaced after as little as a year – typically low value sub £60 elmets worn by scooter boys.
Fibreglass ones have a longer shelf life (check the sticker on the rear – is it blue or green?) but its not the shell you want to be worrying about. If its been indoors, the crucial bit, the liner, made of polystryene and other expanded foams, may be becomming a bit brittle, and its the close fitting of the lid thats so crucial. – The hemet needs a comft but tight fit. If the inner is weak, and moves your head about in the helmet in an accident then its a money saver gone horribly wrong.
Besides, unless things have changed, most agv helmets were pretty cheap and replaceable items at that age.
Given a decent helmet is less than one pair of sports tyres which may last less than 3k on the rear – do you think its a false economy.
bin them, dont sell them, cut the straps out first so no one can “find them” and sell them on.