As of Spring ’16 release, we can now set the Created date field value for the test records in Salesforce test classes using setCreatedDate(Id recordId, Datetime createdDatetime) method.

Example:

@isTest   
private class SampleTest {  
    static testMethod void testSetCreatedDate() {  
        Account acc = new Account(Name='Test Account');  
        Insert acc;
        Datetime yesterday = Datetime.now().addDays(-1);
        Test.setCreatedDate(acc.Id, yesterday);  
        Test.startTest();  
        Account testAcc = [SELECT Id, Name, CreatedDate FROM Account WHERE Name ='Test Account' LIMIT 1];  
        System.assertEquals(testAcc.CreatedDate, yesterday);  
        Test.stopTest();  
    }  
}  

Note:

• All database changes are rolled back at the end of a test. We can’t use this method on records that existed before the test executed.
• We also can’t use setCreatedDate in methods annotated with @isTest(SeeAllData=true), because those methods have access to all data in the org.
• The both parameters (Id recordId, Datetime createdDatetime) of this method are mandatory.