I went with a friend to a meat processor today to pick up a butchered pig. While we were there I saw a sign on the wall that said something like "beef are mouthed upon arrival. If you beef is more than 30 months old, you will have to arrange to pick up the head and spinal cord later."

That's not a perfect quote, but it's close.

I s'pose I'd be considered a city boy, because I have zero idea what that means. What does it mean for cattle to be "mouthed," and why would someone have to pick up the head and spinal cord later?

They look at their mouth, teeth specifically, to age the animal. If it is determined to be over 30 months old there may be some FDA testing required, using the spinal fluid and brain, so maybe "mad cow" disease or other wasting type disease.

The brain and spinal cord have the prions (sp) that cause mad cow disease IIRC. There is some more testing also..

I see. So its sort of, "Come back and get your nasty heads and spines and dispose of them yourself." Right?