Yes, she's way too young for a prong collar. If you can't train her using a flat collar then I would try a front hook harness on her like, a Senseation Harness or the Freedom no pull harness.
I had a strong puller with my pup and it definitely helped with the pulling.
Bear GSD 10/16/11
Elsa GSD 12/23/03 - 11/10/11