Champagne, old but still good?

Assuming that it’s a non-vintage (no production year listed on the label), it will have declined considerably. Most N.V. champagnes are intended to be drunk within a couple of years, and do not improve with age. Chances are it won’t be atrocious, but don’t expect much.

If it’s a vintage Deutz, you could expect to get 10 years out of it.

Oh, and happy New Year!

It could be fine, but @Harp is right that most champagnes aren’t meant to be aged after you buy them. Here is a page on aging champagne. If it isn’t completely sealed it could also have gone flat or picked up weird smells from your refrigerator. Well, the only thing you can do is pop it open and find out!

You’ll know when you taste it. It will either taste fine, or it will taste like vinegar.