At the opening of "Gorin no sho," Musashi mentioned his age as "I have already lived many years and now I'm sixty years old," so if it were exact 'sixty,' he was fifty-nine years old in 1643 when the book was written (in the past, age of 0 was counted as age of 1), therefore, presumably, he was born in 1584.
著書『五輪書』の冒頭にある記述「歳つもりて六十」に素直に従えば、寛永20年（1643年）に数え年60歳となり、生年は天正12年（1584年）となる。