Strange Happenings

Due to some strange activity adults or everyone who turns 21 go missing.
The only living human beings remaining are kids , teens and those below the age of 21.
why 21 ? where do the people go? what happens to them?
To find the answers jump right into the story as two the brothers Taro and Kuro Shirotabi with their friends set out to solve the mystery behind this strange happenings.