D his fifties
in one's fifties表示在五十多岁如果是在他的1岁呢？when he was onewhen he was one year old不是啊，是in one's fifties这个形式啊？？in his 什麼？in one's +几十的复数，表示的是模糊的年龄概念。在某人1岁时，不可以用这个表达的。