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1) a ten years old boy is sitting on the couch.

2) a ten year old boy is sitting on the couch.

3) a ten-years old boy is sitting on the couch.

4) a ten-year old boy is sitting on the couch.

Which is correct?

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Generally, when units of measure are used as adjectives, or as part of a compound noun, they are singular. When expressed as simple nouns, they are plural. Thus,

A ten-year-old boy is sitting on the couch.

The boy sitting on the couch is ten years old.

The boy sitting on the couch is a ten-year-old.

As for the hyphenation, exact usage is a matter of style, but the hyphen will determine how to interpret the phrase.

Ten year-old trees

refers to ten trees which are each one year old, whereas

Ten-year-old trees

refers to some trees which are each ten years old.

  • 2
    Whether to hyphenate at all or not is definitely a matter of style, but I don't imagine there would be many people who'd endorse Ten-year old trees. It looks truly awful to me. – FumbleFingers May 15 '13 at 21:22
  • 4
    Yeah, "Ten-year old trees" doesn't make any sense. It implies that both "ten-year" and "old" separately modify "trees". If you'd edit that example out, I'd upvote this answer as it is otherwise quite good. – WendiKidd May 15 '13 at 21:49
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X year old is a phrase that can be either an adjective, noun or a construct of the verb to be

When used as an adjective the correct thing to say is year old.

A ten-year-old boy is sitting on the couch.

When used as a verb construct it must agree with the noun in terms of quantity.

The boy is sitting on the couch is 10 years old.

You want to use it as an adjective in your sentences so the number 4 is correct.

  • 2
    Complement, I think, not construct. :) – StoneyB May 15 '13 at 20:51
  • @StoneyB: It's certainly a complement in Persian Cat's second example - but unless I'm much mistaken, it would still be a "complement" in, say, "The boy sitting on the couch is a 10 year old." Sure, it's a "noun" as well there, but that just goes to show "adjective/complement" isn't really a useful way of defining the different contexts where singular/plural are idiomatically used. – FumbleFingers May 15 '13 at 21:36
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    @FumbleFingers Too true. It's a wonder anybody speaks the language fluently. – StoneyB May 15 '13 at 21:45

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