The compound "three-month-old" is a modern English construction built from three native Germanic elements. The numeral "three" has a deep Indo-European lineage, ultimately from the PIE root trei-, signifying the quantity. The word "month" is tied to the lunar cycle, originating from the Germanic root for "moon," reflecting ancient timekeeping. The adjective "old," with its sense of age or duration, completes the phrase. When combined in the hyphenated pattern common in English, these morphemes create a precise, formulaic descriptor for age, literally meaning "having an age of three months," which can function adjectivally or nominally.

Structure: three (number) + month (unit of time) + old (adjective suffix) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a cardinal number. - month: From Old English mōnaþ, from Proto-Germanic mēnōþs, related to mēnô ("moon"). Functions as a noun denoting a unit of time. - old: From Old English eald, ald, from Proto-Germanic aldaz. In this compound, it functions as an adjective-forming suffix indicating age.