Set User Role based on age result from Facebook Log-In – WordPress

This assumes that what you said about only needing to check for the minimum value is true. These conditional statements work for this array.

Facebook’s Min Age Array will be either 13, 18 or 21.

$age_range = array( 13, 18, 21); // I don't know what actual data you have here.

if( min( $age_range ) == 13 ) {
    $age_range="under18";
}
elseif( min( $age_range ) >= 18 ) {
    $age_range="over18";
}
else {
    // Unset $age_role if no required values found in $age_range.
    unset( $age_role );
}


if( isset( $age_role ) ) {
    // Here you can assign the role $age_role to the user.
}