How to find age using birthdate...

1. // Save today's date.  
2. var today = DateTime.Today;  
3.   
4. // Calculate the age.  
5. var age = today.Year - birthdate.Year;  
6.   
7. // Go back to the year in which the person was born in case of a leap year  
8. if (birthdate.Date > today.AddYears(-age)) age--;  

This answer from: https://stackoverflow.com/questions/9/how-do-i-calculate-someones-age-based-on-a-datetime-type-birthday

step 1. create a function in SQL server, so that you could use the same logic multiple times.

1. CREATE FUNCTION AGE(@DateOfBirth AS DATETIME,@CurrentDate AS DATETIME)    
2.     
3. RETURNS INT    
4.     
5. AS    
6.     
7. BEGIN    
8.     
9. DECLARE @Years AS INT    
10.     
11. DECLARE @BirthdayDate AS DATETIME    
12. DECLARE @CurrentDate AS DATETIME    
13.     
14. DECLARE @Age AS INT    
15.     
16. --Calculate difference in years    
17.     
18. SET @Years = DATEDIFF(YY,@DateOfBirth,@CurrentDate)    
19.     
20. --Add years to DateOfBirth    
21.     
22. SET @BirthdayDate = DATEADD(YY,@Years,@DateOfBirth)    
23.     
24. --Subtract a year if birthday is after today    
25.     
26. SET @Age = @Years -    
27.     
28. CASE    
29.     
30. WHEN @BirthdayDate>@CurrentDate  THEN 1    
31.     
32. ELSE 0    
33.     
34. END    
35.     
36. --Return the result    
37.     
38. RETURN @Age    
39.     
40. END    

step 2. Call this function using the command, also why do you need a loop, i guess each student have a different V_IssueDate as the current date, so retrieve it on the basis of student Name/ courseName/Job Name.

1. protected void txtdob_TextChanged(object sender, EventArgs e)  
2. {  
3. SqlConnection con = new SqlConnection(this.con2);  
4. con.Open();  
5. SqlCommand cmd = new SqlCommand("select V_Issuedate from StudentRecord where StudentName=@Name;");  
6. cmd.parameters.addwithvalue("@Name",txtName.Text);  
7. DateTime IssueDate= Convert.ToDateTime(cmd.ExecuteScalar());  
8.   
9. SqlCommand cmd2= new SqlCommand("SELECT dbo.Age(@BirthdayDate,@CurrentDate )", con);  
10. cmd.parameters.addwithvalue("@BirthdayDate",txtDOB.Text);  
11. cmd.parameters.addwithvalue("@CurrentDate",IssueDate);  
12. TextBox2.Text= cmd2.ExecuteScalar().ToString();  
13. con.Close();  
14. }