USDA Mad Cow Explanation

I occasionally listened to National Public Radio and as you know there are interviews and debates and question and answer sessions on it on occasion.
About two weeks ago there was a program that had the USDA representative and Creekstone Farms on the program but they were not in a debate but we're in a pleasant Q and A session with the host of the program.
It was interesting what the position of the USDA was. They contend that due to the incubation period of mad cow disease that any cow less than 30 months whole cannot ever have a positive test result for mad cow disease. The fact that Japan and some other countries may be testing, and cow disease in old cattle including those less than 30 months old in the USDA's opinion really gives false assurance since the tested animal made later developed the disease if it was allowed to mature to 30 months of age. They also dismiss the two possible cases in Japan that claim to have a positive results in the house more than twenty months old.
If the USDA is correct, then they have a point. Then the question becomes is there any risk of eating a cow that made later developed mad cow disease but currently is below 30 months of age and has a negative test result. I guess we may find out in fifteen years or so which is how long it takes for CJD to appear in humans from consuming such beef. Or does a negative test result from a cow processed now while it is below 30 months of age be safe enough to even if it was going to develop the disease later?
Of course there will always be the skeptics they claim that the USDA is out to protect the beef industry. I personally don't know if that is true but time will tell.