Because of the limitations on the formula field, this would require creating three fields: “Days Since Closed”, “Days Since Open” and “Age”. Only the “Age” field would be visible on the page layout the others would have to be hidden. 

Formula Fields with Number data type and zero decimal points. Below are the formula values for the fields. 

1. Days Since Closed  = TODAY() - {!CloseDate}
2. Days Since Open = NOW() - {!CreatedDate}
3. Age = IF(ISPICKVAL({!StageName},"Closed Lost"),{!Number_Since_Open__c} -  {!Days_Since_Closed__c},NOW() - {!CreatedDate})