the.com/young adulthood

means The life stage roughly spanning the late teens through twenties when a person is legally adult but still forming the identity, career, and relationships that will define their later life.

from Not a word with a buried etymology so much as a phrase stitched together from two plain English parts: 'young,' from Old English 'geong,' tracing back to an ancient root meaning simply 'youthful,' and 'adulthood,' built from 'adult' (Latin 'adultus,' the past participle of 'adolescere' — 'to grow up,' the same root that gives us 'adolescent') plus the suffix '-hood,' an old English marker meaning 'state or condition of.' The term itself is a modern label, coined by psychologists and social scientists to name a life stage that earlier eras didn't bother separating out — when people moved more directly from childhood to full adult responsibility. So the phrase is young, even if its ingredients are old.