I noticed in pilot that Lorelai tells the guy at the diner that Rory is 16. She wasn't sixteen until a few weeks later when she had her birthday. I would have thought she would have told the guy she was 15. I think the writers had Rory driving by herself, so maybe that was the reason they changed their minds. They probably thought they could keep her in school longer by being a sophomore than turning 17 and being a junior