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.