Theory: はた means 20. For example: 二十歳 はたち、二十人 はたとり、二十年 はたとせ。. ち (個)is a counter for the ひと、ふた、み counting system. Theory (folklore): The 旗乳 (はたち)folktale. During the Warring States period, a young soldier who turned 20 years old wore a banner (旗 はた)of his lords ... More @Wikipedia
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