What’s the best way to query an array in javascript to get just the items from it I want?

You can use pure javascript

var wanted = items.filter( function(item){return (item.age==18);} );

And if your browser does not support the 1.6 version of javascript you can find an implementation of the filter method at https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/filter

Update

Speedwise there is a huge varying ^{(had an error in the test)} difference from a normal loop (depending on browser).. Have a look at this little test i made at http://jsperf.com/array-filter-vs-loop/3