Of course no one actually knows exactly how old each particular tree is - scientists aren't cutting them down to count rings! - instead, membership of the 'club' is determined by a tree's circumference. For an olive tree to be included in the inventory, it must have a trunk perimeter of at least 3.50m at a height of 1.30m above ground. There's more information on this on the Millennium Olive Trees website.
These are the two stunning trees we saw: