try this
<script src="http://code.jquery.com/jquery-1.8.2.js"></script>
<script src="http://code.jquery.com/ui/1.9.1/jquery-ui.js"></script>
<script type="text/javascript">
$(function() {
var date = new Date();
var currentMonth = date.getMonth();
var currentDate = date.getDate();
var currentYear = date.getFullYear();
$('#datepicker').datepicker({
minDate: new Date(currentYear, currentMonth, currentDate)
});
});
</script>
<asp:TextBox id="datepicker" runat="Server"></asp:TextBox>