^^ No, it's nonsense. Friend from way back in high school owns a meat purveyor business, deals with quite a few McDonald's, they use good quality beef. But your comment is amusing on several levels. That you could think the largest restaurant seller of beef in the world could be adequately supplied by using old dairy cows is pretty funny. And even if it were the case, why would it bother you? The only old animals I'm aware of that pose a risk to your health are the very large (and very old) tuna that are usually caught to supply you with maguro (red tuna) sushi; the older fish have accumulated enormous amounts of mercury over their lifetimes.
"Where's the REST of me?"
Ronald Reagan, in "Kings Row"