It depends on the champagne, and depends on the demand at time of bottling. While law dictates that champagne must be aged at least 18 months on its lees, when demand (or perceived demand) skyrockets, producers usually "rush" the aging a bit and send it to stores earlier than they would have previously.

I know that some wine reviewers swear that aging a bottle of Charles Heidsieck Brut Reserve or Veuve Clicquot NV a few years before drinking will take it from delicious to a revelation. Personally, I'm like brendog and can never keep myself (or my SigO) from drinking them far before then. I probably wouldn't age anything lower quality than those two examples above.