JSFiddle - React, Tailwind, and code Playground

HTML

<script>
    function calcAge() {
        var d = document.getElementsByName("day")[0].value;
        var m = document.getElementsByName("month")[0].value;
        var y = document.getElementsByName("year")[0].value;
        if(!d.length || !m.length || !y.length) return;
        var age = ~~((Date.now() - +new Date(y,m,d)) / (31557600000));
        document.getElementsByName("age")[0].innerHTML = age;
    }
</script>
Tag: <input type="text" name="day" onkeyup="calcAge();" /><br />
Monat: <input type="text" name="month" onkeyup="calcAge();" /><br />
Jahr: <input type="text" name="year" onkeyup="calcAge();" /><br />
Alter: <span name="age"></span>