Dear Geist,
Does
. I’m pretty sure I was right but I couldn’t prove it because it isn’t in the dictionary.

Dear Fran,
The phrase
contains hyphens only when they are needed. “My nephew is three years old” is perfectly clear without hyphens. “My nephew, a three-year-old,” and “My three-year-old nephew” are both easier to comprehend with the hyphens. And yes, you’re right—you wouldn’t find the answer in your dictionary unless it has an appendix on punctuation.
—The Editors