Even considering that the first time he was reported in "Shoku Nihongi" (Chronicle of Japan Continued) was after his father, Emperor Konin, ascended the throne even then that he was considered the closest crown prince to the Imperial Throne after Emperor Koken and Emperor Shotoku died (this means that there is no record he was conferred to any position) and his sister, Imperial Princess Sakahito was born when her mother, Imperial Princess Inoe was 37 years old, so we cannot exclude the possibility of late childbearing even it was very rare case at that time.
だが、孝謙天皇称徳天皇亡き後に最も皇位に近い立場にいた筈の他戸親王の『続日本紀』における初出が、父・光仁天皇の即位後であること（つまり称徳朝における叙任記録が存在しない）や姉の酒人内親王も井上内親王が37歳の時の子であることを考えた場合、当時でも稀な高齢出産があった可能性も排除出来ない。