onselect function is not working on my date picker

Onselect is not working on my date picker. Thus it gives me NaN as a return value in other function. Does anybody have an idea or a solution for this? I have no clue.

  $(document).ready(function(){
        var age = "";
        $('#dob').datepicker({
            onSelect: function(value, ui){
                var today = new Date();

                age = (today.getFullYear() - ui.selectedYear)*12;
                age -= ui.selectedMonth;
                age += today.getMonth();

                $('#age').val(parseFloat(age));
            },
            changeMonth: true,
            changeYear: true
        })
    })