The old man opening and closing the movie is Remco Campert, who wrote the novel 'Het Leven is Vurrukkulluk' on which the movie is based. He is credited as 80-year-old Boulie, and thus plays the older version of Géza Weisz's character. It is Campert's first ever role in a movie, and 24 years since his last acting credit in a TV show.
Mees' parents invite him to a recital, conducted by an upcoming conductor, 'Haitink' Bernard Haitink had just joined the Royal Concertgebouw Orchestra as co-conductor in 1961, the year the original book was published. He would move to sole director in 1963 and stay for 25 years, becoming quite famous, but while writing would not yet have been. At the time the movie takes place, he should indeed still be considered 'promising'.