As for what al-Sakuni narrated from Abu Abdillah Imam Ja’far عليه السلام from his father, Imam al-Baqir عليه السلام, peace be upon them both:
Imam عليه السلام said: “The boy, when he is able to fast three consecutive days, then fasting the month of Ramadan has become obligatory upon him.”
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 4, p. 407