AGE OF KOBE BEEF?
When we butcher on the farm, we usually like to butcher them when under 1000 lbs. The age is generally not considered as important as weight. Even regular beef is never fully grown. Typically, a farm will have some bulls and some cows for making calves. These will never be butchered, under normal circumstances, though some will butcher a bull for the toughness and strong flavor.
The cattle that are butchered are generally kept separate, and will be sent to the market at around 2 yrs of age or less. The difference to keep in mind with veal is that they aren't allowed to work the muscles very much, resulting in tender meat. Similar to veal is when a steer or cow will "founder", which is overeating. If a cow founders and survives, the hooves will become deformed in such a way that the cow cannot be active. The beef will then be very tender like veal.
Sorry for the long answer, but I wanted to provide a little more info. Now, the Wagyu actually has more marbling. Marbling tends to be influenced by 3 things: genetics, diet, and activity. Just as with a person, more muscular activity tends to lean out the muscles. What happens in Japan with the Wagyu is that they get very little excercise, along with the specific diet. Any cow could have greatly improved fat characteristics if it were kept penned up and fed a specific diet.
Typically, the conditions the famous Japanese beef is raised in would cause problems in the U.S., due to PETA and other organizations who would take offense to such practices.