SOLUTION ONE
$this->db->where('id', '3');
// เรียกใช้ทุก field ในตาราง
$q = $this->db->get('my_users_table');
$data = $q->result_array();
echo($data[0]['age']);
SOLUTION TWO
// วิธีนี้เรียกใช้เฉพาะ field "age"
$this->db->select('age');
$this->db->where('id', '3');
$q = $this->db->get('my_users_table');
$data = $q->result_array();
echo($data[0]['age']);
SOLUTION THREE
$this->db->select('age');
$this->db->where('id', '3');
$q = $this->db->get('my_users_table');
//ถ้า id คือ key จะเรียกมาได้แค่ 1 row
$data = array_shift($q->result_array());
echo($data['age']);
SOLUTION FOUR (NO ACTIVE RECORD)
$q = $this->db->query('SELECT age FROM my_users_table WHERE id = ?',array(3));
$data = array_shift($q->result_array());
echo($data['age']);