In bottled/ canned beer, flavour changes from oxidation increase over time. Trained brewery taste panel members can actually distinguish between fresh beer and beer that is as little as two weeks old.
Breweries set their ‘best by’ date to reflect the point at which they feel the flavour will have changed so much that the beer is no longer enjoyable. If a beer does not have a ‘best by’ date, assume it will last at least three months from the time it leaves the brewery. Most styles of beer will have deteriorated substantially after six months.
For draught beer, proper beer gas must be used to prevent staling when dispensing. The key beer gas must be used to prevent staling when dispensing. The key beer gas is carbon dioxide which may be mixed with some portion of nitrogen depending on the specific draught system.
Compressed air should never be used to pressurise draught beer. Doing so pushes a large amount of oxygen into the beer and leads to rapid flavour changes.