Specifically, this should do something like you want. Not the most efficient, I'm sure, but it works.
CASE WHEN DATEDIFF(DAY,ProcessStarted,GETDATE()) <> 0 THEN CAST(DATEDIFF(DAY,ProcessStarted,GETDATE()) AS NVARCHAR) + ' days'
WHEN DATEDIFF(HOUR,ProcessStarted,GETDATE()) <> 0 THEN CAST(DATEDIFF(HOUR,ProcessStarted,GETDATE()) AS NVARCHAR) + ' hours'
ELSE CAST(DATEDIFF(MINUTE,ProcessStarted,GETDATE()) AS NVARCHAR) + ' minutes'
END AS 'Age'