Jodie looks so much younger than the other girls

Marilyn (Madge) is 11 years older than Jodie (who was 17 or 18 at the time). Cherie (Annie) is about 3 years older. I don't know about the other girlfriend (Diedre). I have seen a lot of movies over the years that used actors in their 20's & 30's to play teens. Grease is one movie that comes to mind. I am sure the casting agents have their reasons for choosing the actors- whether they are a teens or older.

Jodie was still 16 when this was filmed and Cherie was 19. Both have birthdays in November and neither had celebrated them while the movie was in production in 1979.

Professional Jayne Mansfield fanatic/lover™ since 1980.

In real life, Jodie Foster at that time (or slightly later: fall of 1980) looked really young for her age.

The only actress who looked at all off, age-wise, to me in the movie was Kandice Stroh (Deirdre). Interestingly, she seems to have managed to scrub any reference to her birthdate from her IMDB page, so I have no idea what her actual age is. Marilyn Kagan (Madge) was way older than the character, but didn't look it in the movie. Cherie Curie wasn't much older than the character, and looked right in the movie. There's a contrast between what she looks like all done up and trying to look older, and otherwise, when she looks very much like a teenager.

People in movies are actors and actresses. They usually - almost always - are different from the characters they're playing in all sorts of ways. It works.