Select random from SQL table which evenly distributes across a column's different values, PostgreSQL

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--PostgreSQL 9.5
--'\\' is a delimiter

create table users
(
AGE int,
 ID  SERIAL PRIMARY KEY)\\

insert into users values 
(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),
(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20),(16), (17), (18), (19), (20)\\
\\

with first_random as
(
  SELECT * FROM users where age = '16'  OFFSET floor(random()) LIMIT 20
)
, second_random as
(
  SELECT * FROM users where age = '17'  OFFSET floor(random()) LIMIT 20
)
, third_random as
(
  SELECT * FROM users where age = '18'  OFFSET floor(random()) LIMIT 20
), fourth_random as
(
  SELECT * FROM users where age = '19'  OFFSET floor(random()) LIMIT 20
)
, Fifth_random as
(
  SELECT * FROM users where age = '20'  OFFSET floor(random()) LIMIT 20
)

select * from first_random 
union
select * from second_random
union
select * from third_random
union
select * from fourth_random
union
select * from fifth_random


absolute service time: 0,59 sec

Select random from SQL table which evenly distributes across a column'...