ORDER with CASE and Random for each case

Hi,

If have a view that I would like to sort where I divide the return in 3
different groups.
These 3 groups then should have a random sort order each.
As I am I using it with an offset, and limit, the randomness should be the
same.

For example:
SELECT user_id, age FROM view_users
ORDER BY CASE WHEN age < 20 THEN 1
WHEN age < 50 THEN 2
ELSE 3
END
OFFSET 0 LIMIT 20;

If I have for each age group 30 users. I want these 3 groups to be ordered
randomly but during the paging maintain the order.

The way I would do it now is to use setseed() and a union of 3 selects with
the 3 conditions and random() in each of the three.

Is there a better and more efficient way to do it in one query?

Thanks for any suggestions

A