@tchrist "year-old" is a compound. "fifteen–year-old" is a compound created not by combining the words fifteen, year and old with each other on an equal footing (like "vis-a-vis") where one uses hyphens or where one is a prefix (like "non-English-speaking") where one generally uses hyphens but with one part is already a compound and hence some styles favour the dash to keep the compound ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.