Q:

Why "year" isn't in plural in this phrase?

"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.

A:

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.

