Best Rum Bar?

[Homebrewer]

Excuse my ignorance, but what happens to a 50 year old bottle of rum after another say 20 years? Does it go bad because not barreled? Does it, on the other hand, improve and become more valuable?

I'm guessing but think it doesn't change much after its bottled, perhaps if its stored in a very hot place it might degrade with time but would not improve over time in a bottle