If you ensure that your oil is stored optimally, then it should maintain good flavour and usage characteristics for around 2 years after harvest.
Olive Oil is rich in anti-oxidants, particularly where the olives are processed rapidly after picking. Extensive medical research has shown that anti-oxidants combat free radicals, helping to slow the ageing process and also believed to help in preventing various diseases. Co-incidentally, high levels of anti-oxidants also contribute to the longevity and flavour of Olive Oil.
Olive Oil is known to reduce low-density lipoproteins which is the bad cholesterol in your blood. This improves the ratio of good cholesterol (high-density lipoproteins) to bad which will help in lowering your risk of heart disease.
Extra virgin olive oil is olive oil pressed without the use of heat or solvent treatments. It's the freshest, highest quality Olive Oil you can use and it must be free of any taste defects and have a free fatty acid level of less then 0.8%.
Cold-pressing is where the oil has been extracted using methods that ensure that the temperature never exceeds 27°C. Excess heat is sometimes used because it can increase the quantity of oil extracted, however it also compromises quality.
Olive oil should be stored in a dark location which has a constant, cool temperature, definitely away from direct heat and sunlight. Your pantry or kitchen cupboards away from the stove and the back of the fridge.
Always reseal the container, and where possible store in it's original, darkened bottle packaging.
Try to take your usage patterns into account when you buy your oil to ensure that you're not storing too much - particularly if you don't have ideal conditions.
If you have bought in bulk, decant in smaller amounts for day to day usage - keeping oxygen exposure for the bulk drum / container to an absolute minimum.
If you live in a really hot climate you can store your oil in the refrigerator, in which case you may find your oil will crystallise or solidify slightly. This will not affect the flavour, and allowing the oil to return to room temperature before using will return your oil to normal.