The compound "ten-month-old" is a modern English construction following a transparent numerical-age pattern. Its first element, "ten," descends from the Proto-Indo-European root dekm, which gave rise to the number across Indo-European languages, signifying a complete count. The second element, "month," originates from a Germanic root related to the "moon," reflecting the ancient practice of measuring time by lunar cycles. The final element, "old," stems from a Germanic root conveying the concept of grown or advanced in age. When combined, these morphemes create a precise, formulaic descriptor where the literal sum of the parts—a quantity of lunar cycles plus a state of age—directly yields the modern meaning denoting a specific developmental stage.

Structure: ten (quantity) + month (time unit) + old (age) - ten: From Old English 'tīen', of Germanic origin; related to Dutch 'tien' and German 'zehn', from an Indo-European root shared by Latin 'decem' and Greek 'deka'. Functions as a cardinal number. - month: From Old English 'mōnath', of Germanic origin; related to Dutch 'maand' and German 'Monat', perhaps from an Indo-European root meaning 'moon' (the month being originally a measure of time based on the phases of the moon). Functions as a noun denoting a unit of time. - old: From Old English 'ald', 'eald', of Germanic origin; related to Dutch 'oud' and German 'alt', from an Indo-European root meaning 'adult'. Functions as an adjective describing age.