How did Toshiro Hitsugaya become a captain?

Answer 1

Go on wiki.com, type in his name. Click on the one that says his name and bleach wiki. Go to history and it will tell you there, or just go on bleach wiki.com, type in Tōshirō Hitsugaya, and click history on the list, skip the paragraphs until you find the one that says 'It was there that he met Rangiku Matsumoto', and read on from that.

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Answer 2

Momo Hinamori went to the Soul Reaper Academy first, yes, but due to Toshiro's high level of spiritual energy and his sharp intelligence, he became a prodigy among the other soul reapers, and rose through the ranks much faster than Momo had despite his younger age. This is evidenced by the fact that he has a powerful shikai and bankai, and quickly and correctly suspected Gin Ichimaru of being involved in the tragedies plaguing the Soul Society in the Rescue Rukia arc. He had so much spiritual energy and potential, he had been able to sense Hyorinmaru even before he became a soul reaper.

Simply put, Toshiro rose through the ranks faster. It has nothing to do with when they went to the academy.