How should I name a function which shows or hides an element? I have a function to display or hide some elements. What I want to do is to manage the display of those elements regarding a value.

How should I replace the term toggle? Toggling for me means 'show the element if it's hidden, or hide it if it shown.' (yes, I mean toggling visibility)

function togglePassengers() {
    var nbPassengers = $("#nbPassengersForTravel").val(),
        $passengers = $('#d_passenger').find('fieldset');

    $passengers.each( togglePassenger );

    function togglePassenger(i){
        $passenger = $(this);
        if( ++i &lt;= nbPassengers )
            showPassenger($passenger);
        else
            hidePassenger($passenger);
    }

    function showPassenger($passenger){
        $passenger
            .removeClass("hidden")
            .find("select.age option[value='-1']").remove();
    }

    function hidePassenger($passenger){
        $passenger.addClass("hidden");
        var $passengerAge = $passenger.find("select.age");
        if ( ! $passengerAge.find("option[value='-1']:first-child").length )
            $passengerAge.prepend("&lt;option value='-1'&gt;---------&lt;/option&gt;").val('-1');
    }
}
Naming

I think togglePassenger is not the only naming problem in your code. 

A function parameter should not be a single letter except in a very limited number of cases. But I don't think that this is not one of them; a reader does not automatically understand what i actually does.

I would also rename nbPassengers, because it is unclear what nb means.

What I think your function does is show the first n passengers, and hide the rest. If that is the case, I would not use toggle, because as you said, there really is no toggling going on. You could go with something like updateVisibility to avoid this confusion, although it's a bit unconcrete. Whatever name you do choose, I would add a comment to the function explaining what it does.

Misc


I would use curly brackets even for one line statements to avoid future bugs
your spacing is sometimes inconsistent (eg spacing around variable in function calls)