Very good questions!
So young and old barrels are specifically used for specific blends.
We have heard that a Cognac barrel can be used for a very long time, e.g. there are "forgotten barrels" that can be up to 70 or 80 years old.
Once a cognac barrel gets "too old" it can be used for finishing other spirits (e.g. whisky finishing in Cognac barrels), it can be used for wood chips for barbecue, some people build furniture out of those.
In any case: great topic to think about what happens with a cognac barrel once it retires 😀