Some say that about Mendelssohn, that he was in his compositional prime in his teens (Rondo Capricioso, several overtures, etc). I'm not sure if I agree with that, he had some great works later on as well.
I can't really think of any other examples. Most composers it would seem get a little more experimental as they get older (or their style changes slightly with the changing trends, ie Beethoven, or Rachmaninoff as stated above), and I guess some could interepret this as either more developed or just more strange.