Their ages have never been confirmed. However, Soifon was already serving Yoruichi in the Omnitsokido and appeared to be atleast a teenager when Byakuya was still considered a child, closer to the age of Ichimaru Gin who was most definitely a child at the time, 100 years ago. So even thought the current appearance of Byakuya makes Byakuya look older, based on the Turn Back the Pendulum Arc - it is Soifon who is older.