Q. In the following sentence, is it correct to use an en dash after 25 but a hyphen after 30? “The report referred to a 25– to 30-year-old oak tree on the perimeter of the parking lot.”
A. It looks as if you’re trying to extend the logic behind expressions like “pre–Civil War,” where the idea is that an en dash, which is longer than a hyphen, bridges the space in “Civil War” to apply the prefix “pre” to both words in that phrase.
But Chicago style for your example would be to write “a 25-to-30-year-old oak tree,” with four hyphens. Only in the case of two different ages rather than a range would we recommend something like what you’ve written, but with a suspended hyphen instead of an en dash: “a 25- or 30-year-old oak tree”—which is short for “a 25-year-old or 30-year-old oak tree.” (See CMOS 7.96, section 1, under “age terms.”)
In each of those examples, the hyphen in “25-” is like the one in “30-”; an en dash rather than a hyphen after the first number might look like a mistake, and it wouldn’t necessarily make the expression any clearer. For more on suspended hyphens, see CMOS 7.95.