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  • I am 27 years old
  • I get along with my 28 year old brother / 28 years old brother.

When should I use " year old" and "years old"?

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    "I'm 27 years old", "My 28 year old brother" are correct, but I couldn't tell you why – Sarah May 16 '16 at 20:38
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    My 28-year-old brother is correct. Not my 28 year old brother. It plays the roll of adjective for the word brother. As my teachers taught me. You should hyphenate them. – user33000 May 16 '16 at 20:47
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When used as a predicate adjective, that is, in a sentence that uses this word to describe a noun with a verb like "is" or "seems" or "becomes", you say "28 years old".

When used as an ordinary adjective, that is, preceding the noun as part of the subject or part of the object, you use "28-year-old", singular, with hyphens.

Examples:

"Bob is 28 years old."

"My 28-year-old brother Bob is coming to visit."

Possibly confusing case: "Bob is a 28-year-old man." "28-year-old" here is not a predicate adjective, but an ordinary adjective modifying "man". "Man" is a predicate nominative, but that's not really relevant to the point here.

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    Note that the same rule applies to other measure words. "It is three metres [long]" but "A three metre [long] pipe". – Colin Fine May 16 '16 at 22:35
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    @ColinFine After I made this post, it occurred to me that I should come back and say it applies to other measures. But you beat me to it. Thanks. – Jay May 17 '16 at 6:15
  • Any other mesure word used as an ordinary adjective, should be hyphenate aswell? Like a three-metre pipe? Thanks you all – Liz May 28 '16 at 23:21
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    @liz Yes. "The pipe is 3 meters", but "I found a 3-meter pipe at the hardware store." "There was a 5-minute gap." "He's a one-woman man." Etc. – Jay May 29 '16 at 5:46

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