Edit in JSFiddle

$('#dob').datepicker({
    onSelect: function(value, ui) {
        var today = new Date(), 
            dob = new Date(value), 
            age = new Date(today - dob).getFullYear() - 1970;
        
        $('#age').text(age);
    },
    maxDate: '+0d',
    yearRange: '1920:2010',
    changeMonth: true,
    changeYear: true
});

<input id="dob" type="text" />

<p id="age"></p>

body {
    font-size: 11px;
}