I would be surprised if you ever hear this from a medical professional, from a physical standpoint. Can they throw overhand? Why would they be too young to throw underhand? There are pitching coaches that may not be interested in working with them until they reach a certain age, as they might consider it a waste of time on their part. Expect to make certain concessions to age and physical attributes. 1) Might need smaller ball, for an overhand throw the ball kind of sits on top of the hand, she will need to be able to hold onto the ball as she whips her hand around, might even need to go to something as small as a baseball. 2) Length of workout, and lessons. A 7 year old, might concentrate for 5 minutes one day, and 20 minutes the next. Don't be afraid to cut it short when they just don't seem to be into it. 3) Don't be surprised if she goes in fits and starts concerning lessons and practice, maybe even stopping for several weeks.
There are others out here that know much more about pitching then I do, but imho, I have maintained that by the time they begin playing baseball, kids have thrown thousands of times overhand, (rocks, toys, sticks, all kinds of balls etc), it will take that many times underhand to accomplish the same thing.