Try this:
if ($('#isAgeSelected').is(':checked')) {
$("#txtAge").show();
} else {
$("#txtAge").hide();
}
You can shorten this using ternary, some might say it’s a bit less readable, but that’s how I would do it:
$('#isAgeSelected').is(':checked') ? $("#txtAge").show() : $("#txtAge").hide();
EDIT (14 months later): There’s a much prettier way to do this, using toggle
:
$('#isAgeSelected').click(function () {
$("#txtAge").toggle(this.checked);
});
<input type="checkbox" id="isAgeSelected"/>
<div id="txtAge" style="display:none">Age is something</div>