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  1. A 16-year-old girl.
  2. She is 16 years old.

I've read somewhere that the reason the year in the first example is singular is that it functions as an adjective, and adjectives can't be plural.

Looking at the second example, doesn't the years also function as an adjective? If so, why is it in plural form?

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2  
I would say that in the second version, "16" functions as an "adjective" modifying "years". In the first version there's nothing illogical or ungrammatical about "years" - it's just idiomatic that we normally use the singular. And normally hyphenate the expression, but again that's just a fairly widespread convention in the written form. –  FumbleFingers Dec 4 '11 at 14:47

5 Answers 5

up vote 8 down vote accepted

A 16-year-old girl:
1) "16" and "year" are linked by the hyphen to create a single term ("16-year") that modifies "old."
2) "16-year" and "old" are linked by the hyphen to create a single term ("16-year-old") that modifies "girl."

She is 16 years old:
1) "old" is a predicate adjective for "girl." (as in "She is old.") As a single-word adjective for "she," it is not joined with a hyphen to the previous adjective, "years."
2) "years" modifies "old," and "16" modifies "year. "16" and "years" are not acting as a single adjective for "old," so they, too, do not need a hyphen.

The easy way:
If the adjective string serves as a single modifier before the noun, it needs a hyphen (or hyphens, as in this case). If it is after the noun, it doesn't need a hyphen (or hyphens).

This is the same as "3-day weekend" and "The weekend has 3 days." In the first case, "3" and "day" are linked to create a single term that modifies "weekend." In the second case, "days" is the object of "has," and "3" is a modifier for "days."

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What is happening is that, in

She is 16 years old.

"old" is an adjective modifying "she", and "16 years" is a noun phrase that is acting as an adverb and modifying "old". You can see this by looking at similar sentences:

The festival was a week long.

Here, "a week" is a noun phrase modifying "long". You can tell that it's a noun because you need to put the article "a" before it. When you put the modifier before "festival", you get "The week-long festival". You have to drop the article, because now "week-long" is an adjective.

In English, I believe (now watch somebody prove me wrong) that adjective phrases of this type before nouns cannot be more complicated than the form number-unit-adjective, where the adjective is restricted to some set of adjectives (deep, thick, long, tall, high, wide, late, early, and so on; I don't know whether this class of adjectives has a name). So you can have phrases like

a 1000-foot-deep lake,
a two-hour-late train,
a 10,000-man-strong army.

When they're not immediately before the nouns they modify, you can have much more complicated phrases.

The lake was a strenuous four-hour hike up the mountain.

Here "a strenuous four-hour hike" is a noun phrase acting as an adverb and modifying the preposition "up". If you try to make this into an adjective coming before "lake", you find it's impossible in English. Certainly, anything like "*the strenuous-four-hour-hike-distant lake" is bad grammar.

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I don't think this answers the question. How does this explain why you can't say "a 16-years-old girl"? –  alcas Dec 4 '11 at 17:45
    
I don't think there's anything "definitive" about the presence of hyphens. The adjectival phrase in a "two-foot deep lake" is always the first three words, regardless of whether that first hyphen is present or not, and whether there's another one after "foot". No way are we talking about "two-foot" modifying a deep lake there. It's probably too weak to shoot down @Peter's claim about "maximum 3 elements" in such "compound adjectives", but perhaps you could field a "half-dozen man strong team" in some competition. –  FumbleFingers Dec 4 '11 at 18:57
    
I did hyphenate things incorrectly, though. I'll fix it. –  Peter Shor Dec 4 '11 at 19:11
    
So some ESL websites call them measure adjectives, and claim that there are only seven of them -- tall, deep, high, long, wide, thick, old. But there also seem to be early, late, and strong (in the sense of armies, teams, and so forth). I don't know whether that's a complete list. –  Peter Shor Dec 4 '11 at 22:53
    
Short has been used (whimsically). I'd agree that strong (in the way you mention) is an equivalent usage, but early and late seem to be distinct usages (semantically) in that they invoke an imposed rather than inherent reference frame. Compare below, ahead, away... - they're (spatial or temporal) locatives. –  Edwin Ashworth Jan 14 '13 at 15:58

This comes from the rule of no plurals in compounds - English generally forbids regular plurals (ending with -s) from being followed by another word in a compound. In other words, when you form a compound, only the last word is allowed to be plural.

Thus, "16-year-old", not "16-years-old". For the same reason, a cat who catches rats is a rat-catcher (not a rats-catcher), a table that displays times is a timetable (not a timestable), etc.

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Can you give a couple of examples from other languages? –  Barrie England Dec 4 '11 at 16:52
    
Don't know any offhand; taking it on faith from linguistics articles I've read. I suppose it is not relevant since this is english.se, not linguistics.se, so I think I'll edit my answer to just assert that it's true for English. –  alcas Dec 4 '11 at 17:41
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It's also worth noting that like everything else in English, there are exceptions - "admissions department" and "drinks cabinet" come to mind. The rule holds true most of the time though. –  alcas Dec 4 '11 at 17:43
    
It's probably misleading to call this phenomenon a "rule". Sticking to descriptive terms, it's just a "tendency" - but sometimes in a new unfamiliar context you might use prior knowledge of that tendency to guide your choice of singular or plural. –  FumbleFingers Dec 4 '11 at 18:47

Of numerals, the ‘Longman Student Grammar of Spoken and Written English’ says they 'form a rather self-contained area of English grammar' and goes on to see them as determiners and, as such, similar to quantifiers. By that reckoning, the numeral in the sentence She is 16 years old is grammatically no different from several in the (semantically unlikely) sentence She is several years old.

Of phrases like a sixteen-year-old girl, the 'Cambridge Grammar of English' says ‘Singular forms are used as modifiers before nouns in plural measuring expressions’ and gives as examples a three-month old baby, a five-pound note.

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Actually, in the second example, years functions as a noun, not an adjective. Old is the adjective here. In example 1, the adjective is 16-year-old. When you join words together with hyphens before a noun, you turn them all into a qualifier of the noun that follows, in your example the girl.

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I'm afraid you are confusing cause and effect. Hyphens follow grammar, not the other way round. There are no hyphens in speech. –  RegDwigнt Dec 4 '11 at 14:53
    
You are right. I was referring to the way you write something and I forgot the most important part, speaking. –  Irene Dec 4 '11 at 14:57
    
So, in the second example, "16" and "old" are modifying "years"? –  Sherlock Dec 4 '11 at 15:37
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No, if you look at the parallel example "The train was two hours late", you see that "late" clearly modifies "the train". –  Peter Shor Dec 4 '11 at 15:55
    
I think I may be making the same point as @RegDwight above. Per my link in a comment to the question, the hyphens are optional, and have no effect on the grammatic or semantic roles of either the entire expression *"16 year old", or any of the words within it. –  FumbleFingers Dec 4 '11 at 18:40

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