Mysql Select the same table multiple times with different conditions each time (split a table in categories)

Question

Well, I have this 2 tables:

CREATE TABLE `bookRating` (
  `userId` INT NOT NULL,
  `isbn` VARCHAR(13) NOT NULL,
  `rate` INT NOT NULL,
  `date` DATE NOT NULL, 
  CONSTRAINT `fk_bookRating_user` FOREIGN KEY (`userId`) REFERENCES `user`(`userId`),
  CONSTRAINT `fk_bookRating_book` FOREIGN KEY (`isbn`) REFERENCES `book`(`isbn`)
);


CREATE TABLE `user` (
  `userId` INT UNIQUE NOT NULL AUTO_INCREMENT,
  `country` VARCHAR(3) NOT NULL,
  `age` INT NOT NULL,
  PRIMARY KEY  (`userId`),
  CONSTRAINT `fk_user_country` FOREIGN KEY (`country`) REFERENCES `country`(`code`)
);

I wanted to split the table user into 3 categories. users.age <= 30 , 30 < users.age <= 50 , users.age > 50 and then find the average rating for each category

I figured I could do something like this:

WITH people_age_less_than_30 AS (
    SELECT COUNT(*) count_less_than_30, AVG(`bookRating`.`rate`) as avg_Book_rating_less_30 FROM `user`
    INNER JOIN `bookRating` on `user`.`userId` = `bookRating`.`userId`
    WHERE `user`.`age` <= 30
),
people_age_between_30_50 as (
    SELECT COUNT(*) count_between_30_50, AVG(`bookRating`.`rate`) as avg_Book_between_30_50 FROM `user`
    INNER JOIN `bookRating` on `user`.`userId` = `bookRating`.`userId`
    WHERE `user`.`age` BETWEEN 30 AND 50
),
people_age_above_50 as (
    SELECT COUNT(*) count_more_than_50, AVG(`bookRating`.`rate`) as avg_Book_rating_above_50 FROM `user`
    INNER JOIN `bookRating` on `user`.`userId` = `bookRating`.`userId`
    WHERE `user`.`age` > 50
)
select count_less_than_30, avg_Book_rating_less_30, count_between_30_50, avg_Book_between_30_50, count_more_than_50, avg_Book_rating_above_50
from people_age_less_than_30 join people_age_between_30_50 join people_age_above_50;

This way I know it works! But

I don't think that this is the best way to do what I want to, this is why I am asking, is there an other way to do the same thing? A "better" way?