The compound "three-year-old" is a modern English construction built from fundamental, ancient roots. Its first element, "three," descends from the Proto-Indo-European numeral tréyes, which has remained remarkably stable in meaning across millennia into the Germanic languages. It combines with "year," a term originating from a PIE root for a cycle of seasons, and "old," from a root meaning "grown" or "nourished." The hyphenated structure follows a productive English pattern for forming age-related adjectives and nouns, where the numerical value and unit of time modify the core concept of age, creating a precise descriptor for a specific life stage. The logic is transparently arithmetic, directly linking the count of annual cycles to a state of being.

Structure: three (number) + year (time unit) + old (age) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a cardinal number. - year: From Old English ġēar, from Proto-Germanic jērą, from Proto-Indo-European yóh₁r̥ ("year, season"). Functions as a noun denoting a time period. - old: From Old English eald, from Proto-Germanic aldaz, from Proto-Indo-European h₂eltós ("grown, nourished"). Functions as an adjective describing age.