Many people refer to sex as what someone is biologically and gender as what society considers them (edit: just FYI I'm not an expert and this is probably an oversimplification). While sex is more constant there isn't any reason gender has to be (and there are practical reasons why it shouldn't be but that's out of scope of this comment). This means that while biologically someone might have two XX chromosomes and be 60 years old it's not necessary that society considers them an old woman.