However, the 'Chibanosuke Daidai Gosenzo Shidai' written in a family register of deaths of Hondo-ji Temple has a record, '4th, Tanetsuna, 31 years old, July 8, 1228,' and there is also a problem of how a samurai as young as 14 years old could lead an army in the Jokyu War, and accordingly, there is another argument that he was born in 1198 adopting his age of death being 31, and that Tokitane and Yasutsuna CHIBA brohthers, who were indicated as children of Naritane (younger brother of Tanetsuna) in "Chiba Taikeizu," were biological children of Tanetsuna.
ただし、本土寺の過去帳に載せられている「千葉介代々御先祖次第」には、「第四胤綱　卅一歳、安貞二年戊午五月廿八日」と記されており、また承久の乱で14歳の若武者が一軍を率いるという年齢的な問題もあることから、享年31を採用して建久9年（1198年）生まれとし、『千葉大系図』では成胤の子（胤綱の弟）とされている時胤・千葉泰綱兄弟を胤綱の実子とする説もある。