The compound "thirty-one-year-old" is a modern English construct built from fundamental Germanic elements. Its formation follows a logical, additive pattern common to age descriptors in English: a cardinal number ("thirty-one") modifies the unit of time ("year"), which is then linked to the adjectival state ("old"). Each component has a deep Germanic lineage, with "thirty" deriving from the concept of "three tens," "one" from the fundamental numeral, "year" from a root meaning "season" or "period," and "old" from a root signifying "grown, adult." The hyphenated structure itself is a grammatical innovation that allows this string of words to function cohesively as a single compound adjective or noun, precisely specifying an individual's age.

Structure: thirty (number) + one (number) + year (time unit) + old (age state) - thirty: From Old English 'þrītiġ', meaning "three tens". Germanic origin. - one: From Old English 'ān', meaning "single, one". Proto-Germanic origin. - year: From Old English 'ġēar', meaning "year". Proto-Germanic origin. - old: From Old English 'eald', meaning "aged, ancient". Proto-Germanic origin.