“Young adult,” like many oxymora, is relative and refers generally to a late teen, 20s, or 30s; as opposed to “middle-age” (40s & 50s), and the “elderly” (60s+).
oxymoron: a figure of speech by which a locution produces an incongruous, seemingly self-contradictory effect, as in "cruel kindness" or "to make haste slowly."