Hyphens, En Dashes, Em Dashes

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. Your first version is the right one. “Three-to-four year-olds” would mean three or four children who are a year old.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]