So I think you could do:
<select name="age" class="TEXT90" id="age" onchange="ageUpdate()">
Then somewhere you define the ageUpdate function like:
function ageUpdate() {
var ageList = document.getElementById("age");
var ageSelected = ageList.options[ageList.selectedIndex];
document.getElementById("cat").value = ageSelected.value;
document.getElementById("prices").value = ageSelected.getAttribute("price");
}