Check yourself what your substrings are returning. anyway this can be converted to datetime without any string operations:

SET DATEFORMAT mdy

;WITH CTE_BD

AS(

SELECT *

FROM (

SELECT '06/06/63' Col015 UNION ALL

SELECT '09/17/61' Col015 UNION ALL

SELECT '06/03/69' Col015 UNION ALL

SELECT '11/23/64' Col015 UNION ALL

SELECT '07/10/90' Col015 UNION ALL

SELECT '02/02/69' Col015 UNION ALL

SELECT '09/30/70' Col015 UNION ALL

SELECT '12/05/49' Col015 UNION ALL

SELECT '01/04/67' Col015) AS T)

SELECT CAST(Col015 AS DATETIME), SUBSTRING(Col015,1,2) , SUBSTRING(Col015,3,2) , SUBSTRING(Col015,5,2)

FROM CTE_BD

but be careful because for example '12/05/49' might be converted to '2049-12-05 00:00:00.000'