check whether a checkbox is checked in jQuery

//using plane javascript 
if(document.getElementById('on_or_off_checkbox').checked) {
    //I am checked
} 

//using jQuery
if($('#on_or_off_checkbox').is(':checked')){
    //I am checked
}

$(your_checkbox).is(':checked');

$('#' + id).is(":checked")

if ($('#grepperRocks').is(':checked')) {
	// this checkbox is checked
}

if($('#on_or_off_checkbox').is(':checked')){
    //I am checked
}

$('#isAgeSelected').click(function() {
    $("#txtAge").toggle(this.checked);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js">
  </script>
<input type="checkbox" id="isAgeSelected"/>
<div id="txtAge" style="display:none">Age is something</div>

$('input.checkbox').is(':checked')