It's hard to tell. From what I understand if you take her length from nose to vent her tail would be one inch longer add the two and you would get full nose to tail length. So if 5 inches from nose to vent she would be 11. Then you can look at other dragons to compare and get an idea of possible age. There is a topic here (somewhere) that is all about age, length, and weight.
My guy Toa was 12" at approximately four months.
Hopefully I am remembering that correctly. If I'm off hopefully someone will correct me.