CustomValidator Question

I have a textbox where date of birth is entered. Requirement is that the date of birth be at least 16 years of age. So I have a custom validator with an
OnServerValidate method that does a comprison and returns whether the args.isvalid property is true or false. Now obviously if the method returns a false I want processing to stop and a message to be shown to the user that the dob need to be a least 16 years.

The problem I have right now is not that the code doesn't work right. It does work.(When I set break points and watch it execute, everything seems to run as planned) However, the page just continues to process ! No good ! It just runs through my OnServerValidate method, evaluates to a false, and then keeps going. It eventually redirects to a new page because thats what happens in the subsequent  events. How do Iactually get this to stop processing when the  OnServerValidate method evaluates to false? What am I missing here?

//from .aspx markup

 <asp:TextBox ID="tbDOB1" runat="server" Width="35%"  MaxLength="10" />
 <asp:CustomValidator ID="CheckAgeValidator" runat="server" ControlToValidate="tbDOB1" CssClass="FieldValidatorError"  ErrorMessage="Age must be greater than or equal to 16 years." 
onservervalidate="CheckAgeValidator_ServerValidate"></asp:CustomValidator>

//from codebehind

    protected void CheckAgeValidator_ServerValidate(object source, ServerValidateEventArgs args)
    {
        DateTime enteredAge = DateTime.Parse(tbDOB1.Text);

        // is the person at least 16
        if (DateTime.Now.Date.AddYears(-16) >= enteredAge)
        {
            args.IsValid = true;
            return;
        }
        else
        {

            
            args.IsValid = false;


       } 

        
    }

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