mit zwölf Jahrenspielte er bei eine

mit zwölf Jahren
spielte er bei einem Besuch in Weimar dem damals dreiundsiebzigjährigen Goethe ganze Symphonien von Beethoven auswendig auf dem Klavier vor
und hatte selbst schon fast siebzig Stücke geschrieben.

結果 (英文) 1:

at the age of twelveHe played memorize whole symphonies by Beethoven at the time dreiundsiebzigjährigen Goethe during a visit to Weimar on the pianoand had written itself almost seventy pieces.

結果 (英文) 2:

with twelve years
he played during a visit to the then Weimar dreiundsiebzigjährigen Goethe whole symphonies of Beethoven by heart on the piano before
and had even already written almost seventy pieces.

結果 (英文) 3:

at the age of twelvehe played on a visit to weimar, at the time, all symphonies by beethoven dreiundsiebzigjährigen goethe by heart on the pianoand had almost seventy pieces of writing.