`COUNT` `NULL` issue

Weird issue; for:
```sql
INSERT INTO Item (id, quantity, age) VALUES
  (1, 10, 11),
  (2, 0, 90),
  (3, 9, NULL),
  (4, 3, 3),
  (5, 25, NULL);
```
This:
```sql
SELECT COUNT(age) FROM Item
```
Returns 5; expecting 3
