[NeoCaveman]

No, it ages in the barrel. The spirit interacts with the wood. The glass bottle is inert.

I learned that on the Jameson tour. Although that would have been my guess beforehand.

[SoothingDave]

The fact that distilled liquor goes in clear and comes out brown clues you into something going on in the barrell. ;-)

[SoothingDave]

And they always use fresh oak barrels for bourbon that are charred on the inside before filling. The charring caramelizes the sugars in the wood.