The compound "under-30" is a modern English formation following a straightforward and productive pattern for creating demographic or categorical adjectives. The prefix "under," with its ancient Germanic roots conveying a sense of being beneath or less than a certain point, is combined directly with the numeral "30." This structure logically creates a term meaning "below the age of thirty." While the individual components have deep histories—"under" tracing back to Proto-Indo-European ndher- ("under") and "thirty" to the Germanic base for "three tens"—their combination is a transparent, contemporary construct used primarily in social, marketing, and demographic contexts to define a youth-oriented cohort.

Structure: under (below) + 30 (the number thirty) - under: From Old English under, meaning "beneath, among, before, in the presence of." It functions as a prepositional prefix indicating a position below a limit or threshold. - 30: A numeral representing the quantity thirty, derived from Old English þrītig, from Proto-Germanic þrīz tigiwiz ("three tens"). It functions as the specific numerical limit in this compound.