eighteen-year-old

The compound "eighteen-year-old" is a modern English construction built from three distinct and ancient Germanic roots. "Eighteen" itself is a classic Germanic numeral, formed by combining "eight" and "ten," a pattern traceable to Proto-Germanic ahtōutehun. The words "year" and "old" descend directly from Old English 'ġēar' and 'eald', both with deep Proto-Germanic and Proto-Indo-European origins related to cycles of time and age. The logic of the compound is transparently descriptive, sequentially specifying the number of annual cycles ("year") a person has lived, resulting in their current state of being ("old"), to precisely denote someone at the age of eighteen. This hyphenated structure follows a productive English pattern for creating age-specific adjectives and nouns.

Structure: eighteen + year + old - eighteen: From Old English 'eahtatēne', a compound of 'eahta' (eight) and 'tēne' (a form of ten). Its functional role is as a cardinal number. - year: From Old English 'ġēar', meaning a period of twelve months. Its functional role is as a noun specifying the unit of time. - old: From Old English 'eald', meaning having lived for a long time. Its functional role is as an adjective describing age.