Textbox with calendar in ASP.NET C#

 <td>  Date of Birth <span style="color:Red;">*</span></td>

            <td>

                <asp:TextBox ID="txt_dob" runat="server" CssClass="form-control"  required="" x-moz-errormessage="Enter Date of Birth"

                    placeholder="Date of Birth"></asp:TextBox>

                <cc1:CalendarExtender ID="txt_calender" runat="server" Format="dd/MM/yyyy" TargetControlID="txt_dob">

                </cc1:CalendarExtender>

                <asp:CompareValidator ID="cfvDOB" ForeColor="Red" ControlToValidate="txt_dob" Type="Date" Operator="LessThan"

                            ErrorMessage="(Age can't be less than 15 years)" Display="Dynamic"  ValidationGroup="Save2"

                            runat="server"></asp:CompareValidator>

                            <asp:RequiredFieldValidator ID="reqval" runat="server" ValidationGroup="Save2" ErrorMessage="Enter Date of Birth" ForeColor="Red" ControlToValidate="txt_dob">

                            </asp:RequiredFieldValidator>

            </td>