Mr. Ruge always loved the example of the headline below, in which the hyphen broke off from the printing plate and brought a lawsuit upon the newspaper:
Mr. Jones Has Cast-Off Clothing and Invites Inspection.
Here is a discussion of the issue from Garner's Usage Tip of the Day:
Unhyphenated Phrasal Adjectives
Forgetting to put hyphens in phrasal adjectives frequently leads to miscues. For example, does the phrase "popular music critic" refer to a critic in popular music or to a sociable music critic? If it's a critic of popular music, the phrase should be "popular-music critic."
[[Similar example: He went to a small business convention. Was it a small convention? Or was it a convention for small businesses?] ]
The general rule [Harbrace 18f(1), in the subject field above ] is that when a phrase functions as an adjective preceding the noun it modifies -- an increasingly frequent phenomenon in modern English -- the phrase should ordinarily be hyphenated. Hence "the child is six years old" becomes "the six-year-old child."
Most professional writers know this; most nonprofessionals don't.
Some guides might suggest that you should make a case-by-case decision, based on whether a misreading is likely. You're better off with a flat rule (with a few exceptions noted below) because almost all sentences with unhyphenated phrasal adjectives will be misread by someone. The following examples demonstrate the hesitation caused by a missing hyphen:
o "One last pop on this whole question of incivility of discourse, the much argued over issue of whose speech has been more inflammatory and socially destructive than whose." Meg Greenfield, "It's Time for Some Civility," Newsweek, 5 June 1995. (After "much argued," the reader expects a noun; then "over" appears, unsettling the reader for a moment; then, in two milliseconds, the reader adjusts to see that "much-argued-over" is a phrasal adjective modifying "issue.")
o "This English as a second language text presents the different speaking styles for international students." Mary Newton Bruder, The Grammar Lady 243 (2000). (A possible revision: "This English-as-a-second-language text presents the different speaking styles for international students.")
Readability is especially enhanced when the hyphens are properly used in two phrasal adjectives that modify a single noun -- e.g.: "county-approved billboard-siting restriction"; "13-year-old court-ordered busing plan"; "24-hour-a-day doctor-supervised care." Some writers -- those who haven't cultivated an empathy for their readers -- would omit all those hyphens.
Enlightened writers and editors supply those necessary hyphens -- e.g.: "They lived in a first-floor apartment in a six-story rent-controlled, union-subsidized housing development in Flushing, Queens." Ken Auletta, "Beauty and the Beast," New Yorker, 16 Dec. 2002.