His birthday is December 20, but it has not been revealed how old Hitsugaya is. We know that he entered the Shinigami Academy 5 years after Hinamori, whom he grew up with in Rukongai and he is the youngest captain in the history of the Gotei 13 and his powers have still not matured.. He did not recognize the Vizard, who had disappeared 101 years ago, when the arrived in the fake Karakura Town.
In addition, a Shinigami's physical appearance does not help determine their actual age, as they are older than they look. We know that Rukia is around 150 years old, but we do not know the approximate ages of any other non-Human character.