Science
Why coffee goes stale
The staling mechanisms behind flat-tasting old coffee: degassing, oxidation and aroma loss.
Degassing
Freshly roasted coffee releases carbon dioxide for days to weeks after roasting, a process called degassing. Some CO2 in freshly ground coffee is expected and is part of why very fresh coffee can bloom vigorously; too much CO2 can also interfere with even saturation during brewing.
Oxidation and aroma loss
Roasted coffee's aromatic compounds are volatile and gradually oxidise or evaporate on exposure to air, light and moisture, which is the primary mechanism behind coffee tasting progressively flatter and less aromatic over time, distinct from the drink 'spoiling' in a food-safety sense.
What this means practically
This is the underlying mechanism behind the practical storage guidance in the bean freshness guide: airtight, opaque, cool storage slows oxidation, and grinding immediately before brewing minimises the surface area exposed to air before the coffee is used.