The wheat grass is grown until jointing stage when it is harvested for wheat grass juice. The jointing stage is when the wheat grass is at its highest nutrient phase. After jointing the nutritional levels in the leaves drop and therefore the juice from the leaves at this stage would not be as nutritious. So the plant is very young when it is harvested. As to the number of weeks old, this varies with temperature, rainfall etc. The plants are be closely watched for this critical jointing and harvested then, rather than at a specific number of weeks or height of the plants.