I don't remember when DD5 started but I do know she knew the difference when her sister was born, their 19 months apart. We had many talks about the babies name, she was insisting that her little sister be called "Max" and she didn't care that she wasn't a boy, her name was going to be Max. She continued to call her sister Max until Max started calling her DaDu, that went on until Max was 18 months old and DaDu said "my names not DaDu stop calling me that"! Max reponded, "well, my names not Max" They called each other their correct names from that point on.
