how would they not even know her age before she got on the 70's show? she was in that hbo movie Gia around five years prior to the 70's show.
Lots of actors and models and singers have lied about their age. Maybe the producers didnt see/know about the movie or didnt recognize her in it. Maybe they did but she lied about the age that she was in the movie. They found out half way during the first season and kept her on apparently.
Honestly the older I get the less I am able to gauge the ages of those younger than me lol.