CCC

ASP:

<asp:TableCell>
Birthday:
</asp:TableCell>
<asp:TableCell>
<asp:TextBox ID="BirthdayT" runat="server"></asp:TextBox>
</asp:TableCell>
<asp:TableCell>
<asp:RequiredFieldValidator ID="BirthdayR" runat="server" ControlToValidate="LName" ErrorMessage="Required Field">
</asp:RequiredFieldValidator>
</asp:TableCell>
<asp:TableCell>
<asp:CustomValidator ID="BirthdayV" runat="server" ErrorMessage="Invalid Date" OnServerValidate="BirthdayV_ServerValidate" ValidateEmptyText="True">
</asp:CustomValidator>
</asp:TableCell>

CS:

TextBox:

    protected void BirthdayV_ServerValidate(object source, ServerValidateEventArgs args)
    {
        DateTime selected;

        try
        {
            selected = Convert.ToDateTime(BirthdayT.Text);
        }
        catch (System.FormatException e)
        {
            args.IsValid = false;
            return;
        }

        DateTime min = DateTime.Now.AddYears(-12);
        DateTime max = DateTime.Now.AddYears(-120);
        args.IsValid = !(DateTime.Compare(selected, min) > 0) && !(DateTime.Compare(selected, max) < 0);
    }

Calendar:

    protected void BirthdayV_ServerValidate(object source, ServerValidateEventArgs args)
    {
        DateTime selected = Birthday.SelectedDate;

        DateTime min = DateTime.Now.AddYears(-12);
        DateTime max = DateTime.Now.AddYears(-120);
        args.IsValid = !(DateTime.Compare(selected, min) > 0) && !(DateTime.Compare(selected, max) < 0);
    }