Strangely enough the main issue as I see it is age. Helmets come with expiration dates - some three years, some five, and I suppose some longer but none exceeds seven years to my knowledge. Foam is the culprit. It breaks down over and time and with use. I suppose composite fibers do too. There has also been some serious debate over whether a helmet shell should be rigid or have some give. The research is divided but quite vocal. New materials like carbon fiber have reduced overall helmet weight and are becoming quite affordable. (I picked up a HJC FS-15 in carbon fiber for $330 this year.) Shapes have changed too. My old AGV Ghost was round like a ball compared to my third generation AGV Stealth with aerodynamics aids plastered all over. Ventilation has improved immensely as have shields. There are all kinds of anti-reflective coatings (Iridium comes to mind.) as well as anti-fog coatings. It's a different world for helmets. Other things to consider are: internal helmet shapes, whether or not you want Bluetooth or FM speaker pockets, chin curtains, neck rolls, and which safety standards are important to you.
Short version, to be safe get a new helmet.