correct-va-ages.user.js

// ==UserScript==
// @name           Correct VA ages
// @namespace      http://motemen.github.io/
// @description    Correct VA ages to 17
// @match          *://www.google.com/search*
// @match          *://www.google.co.jp/search*
// ==/UserScript==

var answer = document.querySelector('.answers .answer_predicate');
var knop   = document.querySelector('#knop');
var m;

if (answer && knop && (m = answer.innerText.match(/^(\d+)歳/)) && m[1] > 17 && /声優/.exec(knop.innerHTML)) {
    var correctAnswer = answer.innerText.replace(/^(\d+)歳/, '17歳');
    while (answer.firstChild) answer.removeChild(answer.firstChild);
    answer.appendChild(document.createTextNode(correctAnswer));
}

screenshot.png