The term is a modern English compound formed by combining the preposition 'under', from Old English under (beneath, below), with the numeral '20', representing the age of twenty. The construction follows a straightforward and productive pattern in English for creating demographic or categorical labels, where 'under' denotes being below a specified numerical limit. Its logic is purely arithmetic and literal, directly indicating an age less than the benchmark number, with no abstract evolution from ancient roots required.

Structure: under (below) + 20 (twenty) under: Old English, preposition/adverb meaning "beneath, below, among". 20: English, numeral representing the quantity twenty.