how to calculate age from nvarchar type dob with current system date

Do not store dates as nvarchar data type! Use proper data type[^].

To convert nvarchar data type, use CONVERT[^] function and DATEDIFF[^] to calculate age. Example:

DECLARE @tmp TABLE (dob NVARCHAR(30))

INSERT INTO @tmp (dob)
VALUES('1974-06-13'), ('1999-08-16'), ('2002-02-18')

SELECT CONVERT(DATETIME, dob) AS dob, GETDATE() AS CurrentDate, DATEDIFF(yyyy, dob, GETDATE()) AS Age
FROM @tmp

[EDIT]
If the dob field stores data like '13-06-1974', the conversion will fail if the current server date-time setting is yyyy-MM-dd. In this case you need to change it by using SET DATEFORMAT[^] command. For example:

DECLARE @tmp TABLE (dob NVARCHAR(30))

INSERT INTO @tmp (dob)
VALUES('13-06-1974'), ('16-08-1999'), ('18-02-2002')

SET DATEFORMAT dmy;

SELECT CONVERT(DATETIME, dob) AS dob, GETDATE() AS CurrentDate, DATEDIFF(yyyy, dob, GETDATE()) AS Age
FROM @tmp
WHERE DATEDIFF(yyyy, dob, GETDATE()) BETWEEN 6 AND 14

[/EDIT]