gistfile1.txt

$current_user = wp_get_current_user();
if ( 0 == $current_user->ID ) {
    // Not logged in. Can't comment
} else {
    $birthday = get_user_meta($current_user->ID, 'birthday-your-unique-slug', true);
    if ($birthday >= 18) { 
        //display comment form 
    } else {
        // DO NOT display comment form
    }
}