Set Radio Button From Textbox Input

I have a form where I need to set the radio button according to the
user's input. For example, if the user enters 21 or greater, I need the
adult radio button to be ticked. If the user enters 0-20, I need the
minor radio button to be ticked. Here's my code and I'm sure it's not
right. Please help.

<script language="JavaScript">
<!--
function AdultCheck() {
if (document.form1.AGE.value >= 21) {
document.form1.adult.value.checked = true
} else {
document.form1.minor.value.checked = true
}
}
//-->
</script>