|(Level 1) Adjunct Educator
The most correct answer is to say that philosophy has its origins in human language. Once language emerged, so did the meta-cognition skills needed to reflect on issues such as meaning and ethics etc.
But if you are looking for a more "academic" answer that refers to specific philosophies, then you may need to begin by looking east to the philosophies of Taoism, Buddhism and Confucianism. After that you can look to the Ancient Greeks.
In your case, you are looking to answer this question with respect to a character in a novel. You could answer literally and say that philosophy begins for her when she gets the correspondence course in the mail. But is not the entire book about teen-aged issues and her relationship with an adult who tries to expand her world? I think the origins rest in herself and her willingness to apply the work of past philosophers to her own world.