Whether you should hyphenate “year old” is one of those little grammatical issues that isn’t really an issue at all. This phrase follows the same rules as other compound adjectives, which are two or more words linked with hyphens to show the phrase is part of the same adjective. Except, you know, for when they aren’t hyphenated.
Confused? Don’t be. The rule is straightforward.
If the phrase “year old” is used as a noun or comes before a noun, hyphenate it:
So far, so good. Sometimes, “years old” is used as an adjective phrase that follows the noun, in which case you don’t hyphenate anything. The phrase also becomes plural if the age of the person or object is plural: your computer is one year old, but your friend is thirty years old.
As we noted, hyphenating or not hyphenating “year old” or “years old” is part of a larger grammatical rule governing compound adjectives. The same rule applies if you’re discussing two-seater airplanes, five-foot-tall men, or part-time security guards. For instance, when the compound is used as an adjective preceding a noun, you hyphenate:
You also hyphenate when the compound phrase is used as a noun:
You don’t, however, hyphenate when using a compound adjective as an adjective clause following the noun:
Aw, That Damn Dash: What’s the Difference Between En and Em Dashes?
English Grammar 101: When to Hyphenate Adjectives
http://www.quickanddirtytips.com/education/grammar/are-you-using-hyphens-correctly
http://www.grammar-monster.com/lessons/adjectives_compound_adjectives.htm
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