Hyphens, En Dashes, Em Dashes

Q. Dear Chicago, I’m in a debate with my 73-year-old publisher. I lost my AP Style book in a recent move and I can’t remember the rule for putting hyphens in a person’s age. My publisher says it is only used when the age is a modifier. I say it needs to be used when it is a noun as well, such as: “The healthy 18-year-old jumped in his car . . .” He claims it is only used in a sentence similar to this: “An 8-year-old boy.” Please let me know which is correct so I can end this debate and put this magazine to bed! Thank you, Missouri.

A. Dear Missouri: You are correct. In a phrase such as “a five-year-old,” the age modifies an implicit noun. See CMOS 7.89, section 1, under “age terms,” for more examples.