Hyphens, En Dashes, Em Dashes
Q. What combination of hyphens or en dashes is used to punctuate “a four hundred year old shipwreck”?
A. Please, no en dashes! Write “a four-hundred-year-old shipwreck.” When it comes to compound modifiers, en dashes are useful mainly for expressions that include a compound term that’s always left open: “World War II–era shipwrecks.” In your example, there’s no reason “four hundred” can’t be hyphenated and joined to the rest of the modifying phrase by another hyphen; only rarely, if ever, should a compound contain a mix of hyphens and en dashes. See CMOS 6.80 for more details.