Extra virgin olive oil is fresh-pressed fruit juice, and it has been getting older since the day it was milled. Unlike wine, it does not improve with age — it only declines, slowly losing its aromatics, its peppery polyphenols and finally its edibility to rancidity. The good news: you control most of what determines how fast.
Enter the harvest date printed on your bottle (or your best guess of when the olives were picked) to see roughly where it sits on the freshness curve.
A well-made extra virgin is at its best within about 12–18 months of harvest, and most are still good up to around 24 months if stored well. Robust, high-polyphenol oils (Picual, Coratina, Koroneiki) keep their edge longer than delicate ones (Arbequina, Taggiasca). Once a bottle is opened and air gets in, use it within 1–3 months for peak flavour. This is why the harvest date matters far more than the printed 'best by'.
Oxidation is the killer, and four things accelerate it. Light (keep it in dark glass or a cupboard, never on a sunny counter). Heat (store it away from the stove and oven — a cool pantry, not the fridge, which isn't necessary and causes clouding). Air (keep the cap on tight; buy a size you'll finish in a couple of months). And time itself. Treat oil like a fresh ingredient, not a pantry fixture.
Rancid oil smells flat, waxy or like old crayons, putty or stale nuts, and tastes greasy and dull rather than green and lively. It isn't dangerous, but it has lost its flavour and most of its health benefit. If your oil tastes of nothing — or of staleness — it's time for a fresh bottle.