Wasn't the Hitler Youth mandatory?
Not initially. Beginning as early as the 1920's there were hundreds of youth groups in Germany. After Hitler came to power he consolidated all others into the Hitler Youth. For boys it was pre-military training and for girls it was introduction to motherhood. The main focus for both males and females was discipline and physical fitness in addition to political indoctrination. Because it was possible to pay membership fees but not actually attend the camps, attendance fell to about 25% in 1938. In 1939 the government made attendance mandatory.
Was this youth camp in question mandatory?
The Norwegian youth camp was a gathering of teenagers belonging to a youth wing of Norway's Labor Party. It was political in nature (much like the Young Republicans and Young Democrats in the USA) but in other respects did not function anything like the Hitler Youth. Membership was voluntary.