This is really a loaded question as the answer completely depends on the patients' existing bite. If she has a strong lower jaw already and her bite in the front is "tight" and she has upper spaces between her teeth, for example, than it may be prudent to wait till growth is finished as she may outgrow her final result if orthodontics is pursued too soon. The orthodontist will look into other factors such as family history and possible x-rays of the hand to evaluate growth remaining to make a prudent decision.
Now, if the lower jaw is too far back, then timing treatment during maximum jaw growth (puberty) will help achieve the desired result, and in this scenario, you would want to start treatment. Thus, to answer your question, it depends!