According to the Newsweek profile of him in 2001, Kojima wrote in middle school five 600-page science-fiction novels.
“In middle school he wrote five 600-page science-fiction novels”
Kojima placed his awakening to reading in fifth grade, when he plunged into mystery novels from around the world. He continued with them through sixth grade, and upon finishing elementary school he moved on to science fiction, which he described as his companion until college; from that period he barely remembers reading anything else.
Kojima placed the start of his deliberate narrative writing in junior high, when a teacher assigned converting a few lines of kanbun into modern Japanese and expanding them into a story. He reinterpreted those lines into roughly ten pages, received special praise from the teacher in front of the class, and shifted from merely documenting plots like movie synopses to trying to write real narratives.
At a USC talk in 2011, Kojima said he began writing as a young man and that one of his early pieces was a long story titled Survival Battle, set in a world where everyone fought when they turned fourteen and earned extra time in their lifespan for each victory.
When he read almost exclusively science fiction, Kojima held a grudge against the world, thought a society like that deserved to be destroyed, and preferred books about apocalypse. Upon confronting reality, he changed his reading: he concluded that SF was not a tool for escapism but a medium to send warnings across national and generational boundaries into the future.
According to Newsweek, Kojima got into filmmaking when a friend brought a Super 8 camera to school, and he charged neighborhood kids 50 yen each to see his movies.