Olive oil is not wine — it doesn't improve with age. Extra virgin olive oil is best within the first 12-18 months after harvest.
The harvest year is more important than the "best before" date. Look for the harvest date (raccolta) on the label — it gives a more accurate picture of the oil's freshness.
Old olive oil is not dangerous but loses aroma, polyphenols and health benefits. If the oil tastes flat and greasy, it has seen its best days.