try to use below code..
<asp:textbox id="txtStartDate" runat="server" xmlns:asp="#unknown" />
<asp:image id="imgCalendarStartDate" runat="server" imageurl="~/admin/css/images/Calendar_scheduleHS.png" xmlns:asp="#unknown" />
<ajax:maskededitextender id="meeStartDate" runat="server" masktype="Date" mask="99/99/9999" userdateformat="DayMonthYear" targetcontrolid="txtStartDate" promptcharacter="_" xmlns:ajax="#unknown" />
<ajax:maskededitvalidator runat="server" controltovalidate="txtStartDate" controlextender="meeStartDate" emptyvaluemessage="Start Date is Required" invalidvaluemessage="Date is Invalid" isvalidempty="False" tooltipmessage="Input a Date" xmlns:ajax="#unknown" />
<ajax:calendarextender id="ceStartDate" runat="server" targetcontrolid="txtStartDate" format="dd/MM/yyyy" popupbuttonid="imgCalendarStartDate" xmlns:ajax="#unknown" />