The compound "ten-year-old" is a modern English construction built from three fundamental, ancient roots. The numeral "ten" traces back to the core Proto-Indo-European decimal system via Germanic. "Year" originates from a PIE root for a seasonal cycle or period. "Old" derives from a PIE root associated with growth and nourishment, evolving to signify age. These elements combine in a transparent, formulaic manner in Modern English to create a precise adjectival and nominal compound, literally meaning "having the age of ten annual cycles," showcasing the language's analytical capacity for describing specific age brackets.

Structure: ten (number) + year (time unit) + old (aged) - ten: From Old English 'tīen', from Proto-Germanic 'tehun', from Proto-Indo-European 'déḱm̥t'. Functions as a cardinal number. - year: From Old English 'ġēar', from Proto-Germanic 'jērą', from Proto-Indo-European 'yóh₁r̥'. Functions as a noun denoting a time period. - old: From Old English 'eald', from Proto-Germanic 'aldaz', from Proto-Indo-European 'h₂el-', meaning 'to grow, nourish'. Functions as an adjective describing age.