Trans-2-nonenal (3-Hexyl-2-propenal) gives beer a cardboard flavour. It reminds me of old chip wrappers. Try an out of date bottle of commodity lager if you want to taste it on beer. It is produced by the oxidation of a precursor derived from malt. Trans-2-nonenal is a member of a large group of compounds called unsaturated aldehydes which give beer an old and stale flavour. If you limit the concentration of oxygen in the beer you reduce the rate of oxidation of these unsaturated aldehydes. Brewers who filter yeast out of beer work very hard to keep oxygen to a minimum so that it doesn't damage the flavour.
Anheuser busch used to have the industry's lowest specification for oxygen concentration in beer. Any form of flavour in a cool crisp beechwood sawdust-matured Bud would stick out like a sore thumb! I'm not sure if InBev have continued this tradition. I'm sure les comptables are looking into it. In cask and bottle conditioned beer yeast will absorb oxygen to keep the beer fresher for longer. All beer goes stale and old in the end (yeast seldom survives in the bottle/cask for more than a year or so).
Some drinkers enjoy very old beers which are full of what are described in brewing text books as stale flavours. Some say that they taste like drinking the contents of an pub ashtray out of a prop forward's rugby boot. I find them interesting but aside from lambics which are already just about as oxidised as they can be, I’ve never found a beer to improve after 4-5 years.