Kenji was born in 1960. At that time, Japan was running toward the future. In the time of the economic growth, Japanese children saw a lot of cultural movement. Some are gone, some remain. "20th century boys" this based on that chaos. Kentaro Takekuma, who was born in the same year as Kenji, is going to reexamine the culture of that period.
This is the best guidebook to understand the background of 20th Century Boys.