First off, every child in Germany was forced to join the Hitler Youth. That Grass was thoroughly indoctrinated in their ideas says more about their effective use of propaganda on children than it says about Grass. Then, when Grass was 17, in early 1945, he was drafted and was assigned to an SS Panzer division. He didn't volunteer. At that point in the war, they were sending draftees where they needed them. Grass joined the unit in February of 1945 and was wounded two months later.
Grass spent his life as a holier than thou leftist who conveniently forgot to mention he was a member of the Waffen SS.
As I understand it, toward the end of the war, the Nazis were running out of soldiers, so they were conscripting very young teens and old men.