Dear Geist,

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

Dear Fran,

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.