I had heard that the reason there was a case of Mad Cow Disease in the United States was because the bovine in question was born more than six years ago, before the use of brain and spinal cord in feed was banned, so I got to wondering; How old are most cows before they’re killed for meat?
Darn, and there I was, all ready to point out that the lifespan of slaughtered cows is zero… 
The cow in question was a dairy cow, and therefore probably not slaughtered until it quit producing milk effectively. Steers raised for beef are typically slaughtered at around two years, give or take a few months.
That’s feedlot steers, yabob, which are most of the beef-intended cattle in the country. There’s still a few folks grass-raising them or otherwise not going the feedlot route.
But yeah, even in those cases 36 months is a more typical slaughter age. There aren’t a lot of 4-year old beef cows wandering the U.S.
Dairy cows usually live for around 8-12 years. This is about the average time a cow will successfully breed, and therefore produce milk. Source: my wife, who grew up on a dairy farm.
For the record:
There is almost certainly Bovine Spongiform Encephalopathy (BSE) in the US. There has almost certainly *always beeni/] BSE in the US, as long as there has been a large cattle population.
BSE is a prion disease like scrapie (sheep) or Creutzfeld-Jacob Disease (humans). The agent of all three is a normal neuron protein of that species, not an outside virus or bacteria. Cows get BSE spontaneously, just as humans get CJD. It’s like a slow cancer.
What happens is: rarely, by dumb luck, the protein refolds into a shape that happens to catalyze other copies of the same protien into the same form. (think of crystalization). We have no idea how often this happens, but we do know that we find that in about one person in a million not only does the protein refold, but the catalysis proceeds long enough and extensively enough to slowly cause diagnosable symptoms before they die of, or are impaired by, old age or some other more common cause.
Some (genetic) protein variants are more prone to refolding than others, but even in susceptible individuals, symptomatic CJD exceedingly rare. The process usually takes years or decades to fully evolve to CJD disease.
The big deal about BSE is:
a) we didn’t know the cow, sheep and human proteins were similar enough to catalyze the process in the other species;
b) instead of a single bad luck refolding, the prions may have been amplified by feeding contaminated animals to other animals;
c) we might consume thousands of copies of the altered protein and reach a level that might take years to reach “by accident”
d) the public thinks of ‘mad cow’ as an infectious diease. It isn’t.
Just like 200-300 cases a year of CJD arise spontaneously in the US, I’m comfortable that hundreds of cases arise spontaneously in cows. We just weren’t looking so hard for them before - and since we didn’t know they could trigger catalysis in humans, we didnt care.
If you want proof that it is a public relations panic, consider this: it was estimated inthe year 2000 that direct sheep losses from scrapie in the US added up to $20 million and a national emergency was declared in the sheep industry. We’re not talking a lone sheep, or indirect costs from lost markets (which were tabulated separately), we’re talking about a disease that has been killing thousands of sheep a year for decades, and which has been the target of many industry eradication efforts in preceding decades. We can get CJD from scrapie sheep; our cows can get BSE from meal and supplements made from scrapie sheep.
But let one cow -probably with spontaneous BSE- be found, and the nation is in an uproar,
BTW: you can almost never trace BSE or CJD back to a cause. Any prions in your brain are your own proteins. If they were first catalyzed by a cow prion, it’d have degraded years before you got symptoms. If you find BSE prions in a patient with CJD, they are an irrelevant coincidence (albeit a worrisome one)- the real cause [whether BSE or spontaneous refolding] was years ago, and long since degraded.