1984 Calon Segur St. Esteph

[MoDan20016]

For my parents 18-year anniversary I bought them a bottle of wine from the year they were married: 1984.
The wine is Calon Segur 1984, red Saint-Estephe. My question is will this wine be remotely drinkable (my expectations are quite low) or will the wine be completely past its prime?
And, on the heels of that, what happens to a wine when it's "past its prime?"
Thank You
Monique