The orders displayed in a customer's order history are those they have placed since registering with your store. While you may have customers who have been ordering with your store for years, any orders they placed prior to registering with your store will not be displayed in their order history.

Since Customer Registration was released on August 17, 2010, the earliest possible order that could be displayed for any customer would have been placed on August 17, 2010, provided you enabled Customer Registration for your store on this date, and the customer also registered with your store and placed an order on this date.