"This is a difficult problem for a fifteen-year-old boy."

My guess is that "fifteen-year-old" forms an adjective, but I'm not really sure.

Thank you in advance for your help.


In your sentence

fifteen-year-old boy

is used as an adjective for boy

In the case of

He is a fifteen-year-old.

it is used as a compound noun.

in contrast to

He is fifteen years old

to say how many years he has been alive.

  • Thank you very much. You make me feel I'm on the right track. – Rafiq Mar 9 '16 at 14:54
  • It's a twelve-inch-long ruler. It's a seven-day course of antibiotic. – Tᴚoɯɐuo Mar 9 '16 at 15:04

http://www.grammar.cl/english/compound-adjectives.htm Compound adjectives are written with hypen, and nouns after numbers don't get a plural-s.

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