Here's why:
* Fictional World: Ollivanders is a fictional shop from the Harry Potter universe. While J.K. Rowling provides a lot of detail about the wizarding world, she doesn't explicitly state how long the Ollivander family has been in the wand-making business.
* Legacy and Tradition: It's strongly implied that the Ollivander family has been making wands for centuries, passing down the craft through generations. This is supported by mentions of Ollivanders being "the oldest wand shop in the world" and that the shop is "famous for its wands."
* No Concrete Evidence: There's no official timeline or historical document within the Harry Potter universe to pinpoint a starting date for Ollivanders' wandmaking.
Therefore, while we can infer a long history for Ollivanders, a specific timeframe is not provided within the Harry Potter canon.