BERLIN—A 100-year old man will be tried in Germany on charges of aiding and abetting mass murder while working as a concentration-camp guard, making him one of the oldest defendants in a case brought against alleged Nazi-era perpetrators.

The centenarian was charged with complicity in the murder of more than 3,500 inmates at the Sachsenhausen camp on the outskirts of Berlin. He is alleged to have worked there between 1942 and 1945 as a member of the SS Nazi militia.

The man, who hasn’t been named in line with German privacy laws, was found to be sufficiently fit for trial despite his advanced age and will be able to spend around two hours a day in the courtroom after the trial begins in October, said a spokeswoman for the Neuruppin court where the process will take place.

Historians and rights groups say the case, likely one of the last of its kind, is a reminder that all Nazi perpetrators must be brought to justice irrespective of their age in what should serve as warning to present and future human-rights offenders across the world.

Few suspected Nazi offenders including those in their teens during World War II remain alive. But several were tried recently after a court precedent opened the door to pursuing the lowest-ranking members—who were often the youngest—of the Nazi repression apparatus.

In September, a 96-year-old woman who served as a secretary to the commander of the concentration camp Stutthof is scheduled to be put on trial. The woman, who was 18 at the time of the alleged offenses, has been charged with accessory to murder in 11,000 counts.

Suspected Nazis who worked for Adolf Hitler’s regime at a lower level of command can only be charged with murder or accessory in murder in Germany because the statute of limitation has already expired for offenses such as inflicting bodily harm or unlawful imprisonment.

Many Nazis and their collaborators never faced justice and died in the comfort of their homes in Germany and abroad. Prosecutors often struggle to find surviving witnesses to help build their cases.

Cases against former concentration camp employees who weren’t directly involved in the killings multiplied after 2011, when a Munich court found former guard John Demjanjuk guilty of accessory to the murder of nearly 30,000 people in the Sobibor death camp, setting a precedent.

“We have a duty to do everything to investigate these crimes and to put the people responsible on trial—we owe this to their victims,” said Horst Seferens, spokesman for the Sachsenhausen Memorial and Museum.

More than 200,000 people were imprisoned in Sachsenhausen between 1936 and 1945. Tens of thousands of them died before the camp was freed by the Soviet Red Army. The exact number of victims in the camp, which was also used for training SS guards, remains unknown as the Nazi leadership destroyed the files documenting the mass killing of prisoners before the end of the war, Mr. Seferens said.

Guards such as the centenarian now about to stand trial secured the complex, watched over the inmates and participated in the firing squads.

Prisoners, who included regime opponents, persecuted groups such as Jews, Roma and gay people, and prisoners of war were used as slave labor in a nearby factory. Many were subjected to medical experiments.

At least 13,000 Soviet war prisoners were either shot or gassed in vehicles before the camp was fitted with a gas chamber in 1943.

Remko Leemhuis, director of the American Jewish Committee’s Berlin office, said that such trials had the symbolic power of reminding society that justice must be done as well as warning war criminals of our age that they, too, would face prosecution at some point.

“It is important that the perpetrators are put in front of courts irrespective of their age,” Mr. Leemhuis said.

Write to Bojan Pancevski at bojan.pancevski@wsj.com