如何防止MyBatis查询中出现意外的默认值？

<select id="getUser" parameterType="java.lang.String" resultType="User">
  SELECT * FROM user
  WHERE username = #{username}
  <if test="age == null">
    AND age = 18
  </if>
</select>

public User getUser(String username, Integer age) {
  if (age == null) {
    age = 18; // 设置默认值
  }
  return userDao.getUser(username, age);
}

CREATE FUNCTION getDefaultAge() RETURNS INT
BEGIN
  RETURN 18;
END;

SELECT * FROM user
WHERE age = COALESCE(#{age}, getDefaultAge());