hook_node_grants()

function moderate_node_grants($account, $op) {

$grants = array();

if ($account->age >= 18 ) {
$grants['philosophy'][] = 1;
}

if ($account->age >= 30 ) {
$grants['philosophy'][] = 2;
}

if ($account->age >= 50 ) {
$grants['philosophy'][] = 3;
}

return $grants;
}