how to save the result of a subquery in mysql in order to compare it

Use the same function in the where with a BETWEEN

select u.id, u.nombre, u.rfc as cu, u.nombre as estado, u.categoria, u.fnacimiento, YEAR(CURDATE())-YEAR(d.fnacimiento) as edad
from usuarios as u where YEAR(CURDATE())-YEAR(d.fnacimiento) BETWEEN 19 AND 29

Another way to do it would be to use HAVING , in the following way:

SELECT 
    u.id, 
    u.nombre, 
    u.rfc cu, 
    u.nombre estado, 
    u.categoria, 
    u.fnacimiento, 
    TIMESTAMPDIFF(YEAR, u.fnacimiento, CURDATE()) edad
FROM usuarios u 
HAVING edad BETWEEN 18 AND 30;

• I recommend that you use a more SQL style, with BETWEEN .
• The use of TIMESTAMPDIFF will provide more accurate data, especially when the time matters. If you were born the day X at the hour Y , until the hour Y you have not turned years:)

Test code

SEE DEMO IN REXTESTER

I see two ways (there are more in reality but they are more convoluted)

One: put the same formula in the WHERE

select u.id, u.nombre, u.rfc as cu, u.nombre as estado, u.categoria, u.fnacimiento, YEAR(CURDATE())-YEAR(d.fnacimiento) as edad
from usuarios as u
where ( YEAR(CURDATE())-YEAR(d.fnacimiento) ) between 19 AND 29

Note that I used 19 and 29 because you are using a condition that does not include 18 or 30.

Two: wraps the entire main query as subquery

SELECT * FROM 
(select u.id, u.nombre, u.rfc as cu, u.nombre as estado, u.categoria, u.fnacimiento, YEAR(CURDATE())-YEAR(d.fnacimiento) as edad
from usuarios as u) aux
WHERE edad>18 AND edad<30

Another alternative would be for example to declare a variable and update it in each record to the value of age.

I try to answer from the app I regret but it is correctly indented, but try this

select obj1.*
    from (


select u.id, u.nombre, u.rfc as cu, u.nombre as estado, u.categoria, u.fnacimiento, YEAR(CURDATE())-YEAR(d.fnacimiento) as edad from usuarios as u

    )obj1
    Where obj1.edad >18 AND obj1.edad<30