The term is a modern English compound, constructed using the standard Germanic pattern for describing age. The numeral "twenty-five" originates from Old English twēntig (meaning 'two tens') and fīf (five). The "year-old" component follows the ancient and enduring Indo-European practice of measuring age in solar cycles or "years," a concept linguistically traceable to Proto-Indo-European yēr-. The logic of the compound is transparently descriptive, directly combining a quantity of time ("twenty-five years") with the state of having existed for that duration ("old"). This hyphenated structure functions as a single lexical unit to efficiently denote a specific age category, a grammatical efficiency common across languages.

This is a compound adjective/noun formed from a hyphenated numerical phrase. It is not productively segmented into smaller, independent morphemes in its current fixed form. Its components are the numeral "twenty-five" and the phrase "year-old".