I just had my second Bell Delerium fail. Both developed a crack on the front just left of center. The crack went through the foam, and you could pull it apart a bit. It was purchased 1 year and 10 days ago, so I was told it was outside of the warranty.
But doesn't a cracked helmet indicate that it somehow saved your head from cracking instead? More anecdotal evidence of helmet effectiveness, eh?
To answer the OP, I have no doubt that your older helmet offers the same amount of risk reduction as it did when brand new, for whatever that is worth.