The man from the western district of Borken was a watchman from June 1942 to September 1944 at the Stutthof camp near what was then the free city of Danzig, now Gdansk in Poland.
He was not named by prosecutors, but Die Welt daily identified him as Johann R., a landscape architect who once worked for North Rhine-Westphalia state authorities.
The trial marks a new attempt in Germany's race against time to prosecute surviving Nazis, after a new legal precedent was set in 2011.
The nonagenarian is accused of being an accessory to the murders of several hundred camp prisoners, the regional court of Münster said, more than seven decades after the end of WWII.
These included more than 100 Polish prisoners gassed on June 21 and 22, 1944, as well as “probably several hundred” Jewish prisoners killed from August to December 1944 as part of the Nazis' so-called “Final Solution” operation.
As a watchman aged between 18 and 20 at the time, he is “accused in his capacity as a guard of participating in the killing operations,” Dortmund prosecutor Andreas Brendel told AFP.
“Many people were gassed, shot or left to die of hunger,” he added.
As the guards were a crucial part of the camp system, the man “knew about the killing methods” there, said prosecutors.
'Mentally still fit'
But when interrogated by police in August 2017, the accused insisted he knew nothing about the atrocities in the camp, Die Welt reported.
Asked why the camp detainees were so thin, the defendant reportedly said food was so scarce for everyone that two soldiers could fit into a uniform.
The defendant will make a statement during the course of the trial, his lawyer told national news agency DPA.
Stutthof was set up in 1939 and would end up holding 110,000 detainees, out of which 65,000 perished, according to Museum Stutthof.
The defendant will be tried before a juvenile court as he was not yet 21 at the time of the crimes.
Given his advanced age now, each court hearing will likely last for a maximum of two hours.
“But mentally, he is still fit,” said Brendel.
If found guilty, he risks up to 15 years in prison, although the elderly man is unlikely to serve any time.
Nevertheless, Brendel said “if one looks at how many evil doings and crimes were perpetuated, one can understand why elderly people too have to face prosecution.
“Since there is no statute of limitations in Germany on murder, as a prosecutor one is obliged to pursue this case. But the moral aspect should also be considered,” he said.
“Germany owes it to the families and victims to prosecute these Nazi crimes even today. That is a legal and moral question.”
Germany has been racing to put on trial surviving SS personnel, after the legal basis for prosecuting former Nazis changed in 2011 with the landmark conviction of former death camp guard John Demjanjuk.
He was sentenced not for atrocities he was known to have committed, but on the basis that he served at the Sobibor camp in occupied Poland — for having been a cog in the Nazis' killing machine.
German courts subsequently convicted Oskar Gröning, an accountant at Auschwitz, and Reinhold Hanning — a former SS guard at the same camp for the mass murders.
But both men, convicted at age 94, died before they could be imprisoned.
Prosecutors have also filed charges against another former SS guard at Stutthof, a 93 year old from the city of Wuppertal. It remains to be determined if he is fit to stand trial.
Historian Peter Schöttler underlined “an important humanitarian and legal reason” to push on with the justice process.
“If we let this case go, then there will be a new excuse for letting something else go. A rule of law should not allow for exceptions.”