Les comparaisons

<p>Introduisez votre âge</p>

<input id="age" value="18" />

<button onclick="myFunction()">OK</button>

<p id="demo"></p>

<script>
function myFunction10() {
var age,voteable;
age = document.getElementById("age").value;
voteable = (age < 18) ? "Trop jeune pour":"En âge de";
document.getElementById("demo").innerHTML = voteable + " voter !";
}
</script>