Case When Statement Age not calculating when Archived_Date is NULL and returning NULL value in Age column

CASE    
    WHEN Isconverted = 'TRUE'    THEN DATEDIFF(DAY, Task_CreatedDate, OPP.CreatedDate)
    WHEN Isconverted = 'FALSE'   THEN DATEDIFF(DAY, Task_CreatedDate, A.Archived_Date)
    WHEN A.Archived_Date IS NULL THEN DATEDIFF(DAY, Task_CreatedDate, GETDATE())
    ELSE 'NULL'
END AS 'Age'

** Above case when statement is not calculating third line when Archive_Date is null I need the age column to be populated with current date – Task_CreatedDate **

Answer

I think you want:

(CASE WHEN Isconverted = 'TRUE'
      THEN DATEDIFF(DAY, Task_CreatedDate, OPP.CreatedDate)
      ELSE DATEDIFF(DAY, Task_CreatedDate, COALESCE(A.Archived_Date, GETDATE()))
 END) AS Age