如何在查询中按年龄分组显示空数据？

CREATE TABLE User (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age_id INT
);

INSERT INTO User (id, name, age_id) VALUES
  (1, 'Alice', 1),
  (2, 'Bob', 2),
  (3, 'Charlie', NULL),
  (4, 'David', 2),
  (5, 'Eve', 3),
  (6, 'Frank', 3);

CREATE TABLE Age (
  id INT PRIMARY KEY,
  age INT
);

INSERT INTO Age (id, age) VALUES
  (1, 20),
  (2, 30),
  (3, NULL);

SELECT Age.age, COUNT(User.id) AS user_count
FROM Age
LEFT JOIN User ON Age.id = User.age_id
GROUP BY Age.age;

+------+------------+
| age  | user_count |
+------+------------+
| 20   | 1          |
| 30   | 2          |
| NULL | 2          |
+------+------------+