A cliché popular with literary schoolmasters when I was a boy held lyric poetry to be the province of youth, and prose fiction of maturity. Exceptions to the rule such as Yeats or Radiguet were said to prove it.
Like all clichés, the distinction, though crude, contains a germ of truth if applied in context, depending as it does on a Romantic notion of imagination as a living organism, itself subject to the process of aging. To adapt the image of that archetypal romantic poem, Kubla Khan, genius erupts in an untameable flood of song from the young (Shelley, Byron, Keats), subsiding into a sedate river of prose in middle-age (Austen, Scott). Nonsense, perhaps, but suggestive nonsense.
It may be that an accident of history gave birth to the cliché in the first place, not least the early deaths of so many brilliant poets in an era enthralled by myths of revolution and renewal. The chronology is also questionable: one might be, in the words of another