Thin:
Genetics wouldn't cause hair loss at age 13. Dandruff, despite its reputation, never causes baldness.
Do you pick and scratch at the hairless area? If you do, have you always? That would indeed explain the problem.
I suggest you show your scalp to a dermatologist, who should be able to sort out the various possibilities, contributory or otherwise, and quite possibly might be able to recommend treatment.
meanwhile if you can suppress the picking by noticing and making yourself stop, do it. But that may not be easy.
Best.
Dr. R