The theory is based on what Masatake KOMAI wrote about the dispute over the head of the Imagawa family in his diary about the day of Shingen experiencing his first battle as having been at the age of 16.
その理由に、信玄が16歳にて初陣に出たと言う輝かしい日に、駒井政武は日記に、今川家の家督争いを書いている。