I know that Cognac and Scotch dont age as such in the bottle but at what point do they start to fall apart ? ie if you have a 50 year old cognac how long can it survive in the bottle before harm due to old age sets in ? 10 years ? 20 ? 50 ?
If stored properly, and seal is good, I think they can last a long time. A couple years ago I stumbled on a Mortlach 16 I had purchased in Edinburgh back in 2001. It was stellar (wish I could get more of that bottling). That’s only about 15 years, but at least one data point for you. I have Cognac I store with my wine in a locker that I certainly hope is not deteriorating.
In a sealed full bottle I wouldn’t have concerns.
1/10th of a bottle left on a warm restaurant shelf and I’m not buying.
Odds are that you’ll go bad before the Cognac does.
The few drops I got from a tiny bottle of 1875 were very good.
Had a 40 year old 2 weeks ago that was a wowzer !
Had some 1910 labaude that was great
I follow the same rule for all my spirits - more than 1/3-1/4 of bottle, do nothing (other than keep out of direct light). Less than that, either break down into smaller vessel or drink within a month.
No. It starts bad and doesn’t change 
Dan Kravitz
Anyone have spirit storage in their cellar? Thinking of adding shelves for my whiskey collection for my cellar build.
Going back to the OP, which was a bit ambiguous. First question. How long does a spirit last in bottle before it is opened. My assumption is that this is forever (or effectively that for those of us who will not live past 120). The second question. Once a bottle is opened, is there a shorter life span than if it were not opened. And, to pick up on William S’s point, does it matter if it is less than 1/2 or 1/3 full. I have some armagnac that were opened a while ago, and that seem fine, but to be honest I do not have enough experience to compare to those that were not opened until recently. So does this make a difference? And what I really mean here is does it make a difference over a 100 year span? Or 50? Or 25?
I have never had a high proof spirit that was bad. Have opened lots of booze: gin, scotch, American whiskey and then tons of old Amari. The Amari, at 16-20% and wine-based, Do oxidize once they are opened, and any spirit left open for long Times is going to lose aromatics to evaporation and oxidation. But a well stored bottle, unaopened, of a high proof spirit, should basically be like a time capsule.
+1 What Noah said !
“bad”? no. Everything experiences oxidation.
Cognac does develop in the bottle, some time for the better, but at a glacial pace. I’ve had bottles that were bottled in the 19th century that were almost certainly not negatively affected by their time in the bottle. Of course it needs to be properly stored; too hot and the alcohol will evaporate, too cold and the cork will dry out. An old bottle with a reasonable level, stored in reasonable conditions, will not go bad, at least not for many centuries. I’ve also had bottles that have been open for many years that have been just as good as when first opened. Again storage is key; I would normally also reseal with a fresh cork if I knew I wanted to keep it for more than the immediate future.
I probably like old cognac as much as if not more than wine. So i’ve spent a good bit of my energies on collecting and drinking old spirits.
In my experience, cognacs definitely do age in the bottle, as some have suggested. It may indeed happen very slowly, but it does occur. There is no other way to explain the complexity and magic of so many old 200+ yr cognacs that were clearly put in bottles relatively early and have spent 3 lifetimes in the bottle since. The idea they have been “frozen” since being bottled is obviously nonsense.
Unopened bottles will last a very, very long time. Even primitive bottles from the late 1700s and early 1800s are still generally sound. As with wine, not every bottle is perfect, and some are dead right off the bat. But i’d say my batting average with 200 yr old bottles is still 70%, and I attribute the bad bottles more to seal issues than the spirit running out of gas, so to speak. How long a modern bottle with a better seal will go is a question future generations will have to answer.
Once a bottle is open, as others have said, you need to be careful once the level gets low. When ordering individual glasses, for example, bottles lower than, say, 1/3rd or 1/4th full are bad much more often than fuller bottles. And unopened bottles have the highest hit rate.
Rule of thumb: if the bottle looks good, your odds are pretty high of having a delicious experience. For cognac, 1811 remains in a class of its own. 1812 and 1807 are also excellent, as are 48, 65, 75, 80 88, 98, 00, 04, 10, 22, and 45 on the younger side.
I’ve heard there is some old Cognac in pharaoh’s old tombs…
Darrell Corti is a big believer in spirits ageing in the bottle so he poured us some spirits that had been bottled before Prohibition. I agree with him but it s a long wait!
I have never left a bottle of Cognac opened long enough to see what happens.
\When I worked with a cooperage then owned by Hennessy I got to visit Paradis several times…they keep Cognac in barrel for over 100 years, then put it in ceramic.
Patrick, where do you buy bottles over 100 years old??