Even if he were only 15, that would still make 1891. Yellowstone opened as a national park in 1872, with a thousand vistors that year. You could get there a combination of rail and stagecoach by 1880. Rudyard Kipling wrote about a fly fishing trip there in the 1890's (I believe) as though it were a regular thing. I seriously doubt that Hewitt was the first to fly fish in the park (although he may have believe that he was.)