The compound adjective "week-old" is a transparent and relatively modern formation in English, following the productive pattern of combining a unit of time with the adjective "old." Its first element, "week," originates from the Old English concept of a cycle or succession, reflecting the ancient human observation of time in recurring periods. The second element, "old," stems from a common Germanic root conveying age and the passage of time. The logic of the compound is directly literal, combining these two elements to precisely denote an age or duration of exactly seven days, a construction that efficiently serves descriptive needs in contexts ranging from journalism to everyday conversation.

Structure: week (period of seven days) + old (having existed for a specified time) - week: From Old English wice, wicu ("a turning, succession, change"), of Germanic origin. Functions as a noun specifying a unit of time. - old: From Old English eald, ald, of Germanic origin. Functions as an adjective describing age or duration.