DECLARE @String VARCHAR(10);
DECLARE @DateValue DATE;
SET @String = '250809'; 

-- Convert your undelimited string DDMMYY into a DATE
-- First: Add / between the string parts.
SET @STRING = SUBSTRING(@String,1,2)+'/'+
     SUBSTRING(@String,3,2)+'/'+SUBSTRING(@String,5,2);
-- Second: Convert using STYLE 3 to get DD/MM/YY interpretation
SELECT @DateValue = CONVERT(Date, @String, 3);

-- Using the DATE 
-- Select the value in default Year-Month-Day
SELECT @DateValue AS DefaultFormat;    
-- Select the value formatted as dd/mm/yy
SELECT CONVERT(VARCHAR(20),@DateValue,3) AS [DD/MM/YY];
	
DefaultFormat
-------------
2009-08-25

DD/MM/YY
--------
25/08/09
	
DECLARE @d CHAR(6) = '250909';
SELECT DATEFROMPARTS('20'+RIGHT(@d,2),SUBSTRING(@d,3,2),LEFT(@d,2));