Everyone knows that whiskey doesn't age in the bottle. That sealed bottle of 1964 Very Very Special Old Fitzgerald you "found in your grandfather's basement" should taste the same today as it would have 50 years ago. This is a good thing. It's what makes dusty hunting possible. Aging, however, is different from oxidation.
Once the bottle has been opened and the contents have been exposed to oxygen, the contents of the bottle do start to change. Normally, this isn't an issue. Most people don't have bottles that sit open for decades. In many cases, oxidation is in fact a good thing. It's the reason that a bourbon can improve on the second or third tasting.
In Japan, however, oxidation can be a problem. At whiskey bars, the price per pour is often very high and, as bourbon is less popular than it once was, open bottles tend to languish.
For example, I've tried the "cheesy" gold foil Wild Turkey 12 Year on several occasions with different results. I use this as an example because I drink a lot Wild Turkey 12 Year bottled in the 2000s and so I am familiar with the general taste profile. The first time I tried the "cheesy" gold foil version, the bottle was relatively full and the bourbon was great.
On two other occasions, however, the bourbon came from almost empty bottles. This time, the bourbon was relatively flat and boring. It lacked the complexity and character that I had expected. I've experienced the same thing with Van Winkle Family Reserve Bourbon bottlings from the early 1990s and with the older bottlings of A.H. Hirsch. It's very likely that all of these bottles had been sitting open on the shelf for more than a decade.
This is all to say that when ordering rare older bottlings at a bar in Japan, consider how long the bottle has been open. If you're getting the last little bit of the bottle, you might not be getting a good example and may not get to experience why a bourbon is thought to be so great.