in his one不对,in his fifties是对的
in one's 加"几十",意思是某人几十多岁的时候的意思,如in his sixties (在他六十多岁的时候),in her thirties
(在她三十多岁的时候),如果说在他一岁的时候,应该说成when he was one 或是at age one