Missy,
It's too bad they goofed and didn't notice your birthdate before they hired you. What a disappointment! I don't know about where you live, but here in Alberta, the regulations for time and hours worked is quite different for 15 and under and 16 and up. Many employers are reluctant to hire anyone under 16 because of the hassle. Around here, the clothing stores seem to hire more of of the under 15 year olds, because they are not open after 9pm.
You'll get another, better job, I'm sure.