Q. I am copyediting a parent resource website and this comes up a lot. How do I hyphenate “this class is for three- to four-year-olds”? Is that correct? I have seen it as: three-to-four year-olds.
A. The first one works well enough, but Chicago would apply an additional hyphen: “three-to-four-year-olds.” “Three-to-four year-olds” would mean three or four children who are a year old. See also CMOS 7.96, section 1, under “age terms.”