When did Natalie Dorris buy her first root beer?
when a customer bought their first root beer refers to FirstPurchaseDate;
SELECT T2.TransactionDate FROM customers AS T1 INNER JOIN `transaction` AS T2 ON T1.CustomerID = T2.CustomerID WHERE T1.First = 'Natalie' AND T1.Last = 'Dorris' ORDER BY T2.TransactionDate LIMIT 1