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I hate to be the bearer of bad news, but there's a common misconception that's been permeating the fandom, even on the official wiki, that's just...not true.
Dazai met Akutagawa when he became an executive, which happened when he was 17-18. In both The Heartless Cur (the short story written by Asagiri about how Akutagawa and Dazai met) and Beast, which follows the original timeline, albeit with some changes, it flat out states that the interaction was only four and a half years prior to the main story. Which means Akutagawa knew Dazai for six months. He was already either fifteen or sixteen when they met, and they haven't known each other for that long. Not fourteen.