Apostolidis was born in Athens in 1924. His father was Heracles N. Apostolidis, who was a journalist, chief editor in several newspaper agencies, head of "Encyclopaedia" of Pyrsos publishing, head of the National Library (1945–1959) and creator of the first "Poetry and Short Stories Anthology." His mother, Elpiniki, was a teacher. After primary school, he attended Varvakeios High School (1935–1941), where on 28 October 1941 organized student abstention. Apostolidis participated in the banned, by Papandreou government, march on 3 December 1944 with EAM followers. From 1945, Apostolidis studied in the department of History and Archaeology of the University of Athens, but was forced to suspend his studies in order to pay his duties in the Greek Army during the Civil War, where he participated as Second lieutenant of the National Army. Living so close with destruction and death, Apostolidis stated that he swore to himself not to shoot a single bullet, but to record what he was going through for two and a half years in Grammos, Vitsi and during the cleansing operations of Roumeli and Peloponnese. As soon as he was released from his duties, he published "Pyramid 67", a text about the Civil War. He was dismissed characterized as a "second rank nationalist."