It's hard to say just from the size. Pet stores are notorious for not feeding their dragons enough or otherwise caring for them poorly, so she could very well be an adult. Or, if she's been getting proper care then she could be a juvenile with a bit of growing left to do.
Did you just get her, or have you had her for a while? If you know she's been fed properly and kept in a healthy environment with proper UVB, calcium supplements, etc. I would say 6-8 months. If you just got her at 15 inches, she could be 5 years old, there's no telling.
Correct. But just to be sure....we should really see a pic of the sweetie :wink:
No, that sounds like a Juvenile...I'm just trying to get a pic out of you.