+-----+--------------+-------+
| Id   |  FirstName  | Age   |
+------+-------------+-------+
|   1  | Kate        |  100  |
|   2  | Misha       |    0  |
|   3  | Nick        | NULL  |
|   4  | Larisa      |  200  |
+------+-------------+-------+

SELECT AVG (Age) FROM Persons