is not wine -- it's a type of brandy from a particular region of France that's made in a particular manner.
Unlike wine, spirits, including Cognac, do not improve once they're bottled, and in fact they begin to deteriorate, especially if exposed to heat and light. So the age of your BIL's bottle would not affect its value, unless it were a very rare or one-time bottling (e.g., Courvoisier Erte
-- which just happens to be made form vintages that on average have been aged for 60 years before blending).
Any added value would be due to the rarity of the bottle, however, not because the booze in it has gotten any better.
BTW, the empty bottle of something like Courvoisier Erte has value -- don't toss it out!