addEvent(window,'load',ConfigureAgeSelect);

function ConfigureAgeSelect() {
    oAgeSelect = document.getElementById("Field07");
    oAgeSelect.onchange = function() {
        SelectAge();
    }
    SelectAge();
}

function SelectAge() {
    oAgeSelect = document.getElementById("Field07");
    if (oAgeSelect.value == "<18") {
        document.getElementById("AgeGroupMessage").style.display = "block";
    } else {
        document.getElementById("AgeGroupMessage").style.display = "none";
    }
}