Q. Which is correct: The 20-million-year-old scroll or the 20 million-year-old scroll? – from Tucson, Ariz. on Fri, Feb 20, 2015
The second, though prehistoric dates are better phrased without a
compound modifier: an object estimated to be 20 million years old. That
long predates humans so it couldn't be a scroll.
With respect to our friends at Ask the Editor (who, to their great credit, introduce the annual revisions to the gospel in front of a room full of rabid style nerds, which takes no small amount of sand) -- no. That's just wrong, and it's wrong on a lot of levels, so stop it before you do any more damage to the cause of style.
AP style, like many, exempts number compounds of the million, billion and percent flavor from the general rule on hyphenating preposed modifiers -- reasoning, quite sensibly, that "$7.82 billion budget" and "4 percent increase" simply don't create the sort of ambiguity you get from "man eating blancmange" vs. "man-eating blancmange." The problem comes when the little compound turns into a bigger compound: in the example I still use in class, when a dozen photos taken 20 or so years ago become "two decade-old photos." It's charitable to call that one ambiguous; the word you want is "wrong."
You could always write a rule that says "use hyphens to be clear" and count on the grownups in the room to figure it out on the fly. They generally do, but human beings -- especially copy editors -- are rule-loving creatures. If you're going to write a hyphen rule, write one that isn't likely to paint you into a corner: "Hyphenate the whole damn compound."
While you're at it? "Prehistoric" doesn't seem like a very intuitive cutoff point for when a bunch of numbers ought to be clumped into a compound. And if you think something couldn't be a scroll because it predates humans and their silly artifacts, you haven't been paying attention.