In 1866 Grünfeld was appointed court violoncellist to King William of Prussia.[עריכת קוד מקור]
it was translated from the english wiki. but at 1866 he was 11 years old!
I agree it seems odd, but this is what is written in the source, the Jewish Encyclopedia. I think that maybe in the 19th century it wasn't as unthinkable as it is now for a child of 11, if he was especially talented, to work in a court orchestraשלומית קדם - שיחה 19:08, 11 בדצמבר 2010 (IST).