By default the MinDate for RadDAtePicker is "1/1/1980 12:00:00 AM". So, you need to set lower date value than the actual value populates in datepicker.
Set the MinDate for the RadDatePicker as shown below.
<telerik:RadDatePicker ID="RadDatePicker2" runat="server" MinDate="01/01/1800">
</telerik:RadDatePicker>