What we know as whisky maturation is a dance between 4 interrelated processes - infusion, evaporation, oxidation, and other chemical reactions. These all happen together, and very nicely, when whisky sits in oak barrels for an extended period of time.

Colour, and oak flavour are infused into the whisky simply by sitting in the barrel. The whisky will slowly evaporate while inside the barrel as well. Volatile compounds evaporate, making the whisky smoother, deeper, and more complex with age. Fascinating chemical reactions happen between compounds in the wood, and in the whisky. As ethanol degrades lignin, for example, it creates new compounds, which themselves interact with other molecules and compounds in solution.

The age statement on a bottle of whisky refers to the time it spent in a barrel, doing those lovely things.

Common wisdom is that the whisky is done changing when it goes into glass. Certainly, infusion and evaporation are finished. But! Oxidation, and reactions between compounds in the whisky itself will continue, even in a sealed glass bottle. Usually this happens too slowly to notice, or the bottle gets drank before a change can be observed, but change certainly happens.

Long story short - whisky won't go bad. In fact, sometimes it even goes 'good'! I had the chance to try a young single malt, that was bottled in the 1970s. It was wonderful, and had signature aroma and flavour characteristics of a very old whisky. This is due to slow oxidation, and the glacial interaction between esters and congeners over time, which will happen no matter what vessel the whisky is in.

Whisky that has been exposed to too much oxygen, like if the bottle sits nearly empty for a long time, or has a bad seal, will often end up tasting flat and bland. But 'good' or 'bad' at this point, is a subjective matter. Only one way to find out!