This has come up before - you're not dreaming . It might be interesting to see what the manufacturers have to say on the subject, but of course they will be interested in selling you a new helmet as frequently as possible
However, I don't think that polystyrene foam is going to degrade seriously unless you left the helmet outside in the sun and rain for the last 9 years - at which point it would be crumbling. Take a close look at it and see what you think. In the event of falling on your head, it will likely still offer the protection it did when new.

Ed