PLSQL transfer modify data

[Manos]

Hi am trying to convert some birth dates from one table to another as age groups

to be more spesific i got one table "xsales_customers"

 

and i want from that table to transfer and modify the data from the column birth_date to another table named customers as agegroups ie under 35, above 70 etc..

here is what i have done so far

CREATE OR REPLACE PROCEDURE get_age_group AS
BEGIN
FOR omada27 IN (SELECT id FROM CUSTOMERS)
LOOP
UPDATE CUSTOMERS
SET Agegroup = TRUNC(MONTHS_BETWEEN(SYSDATE, BIRTH_DATE))/12
 SUM(CASE WHEN agegroup< 18 THEN 1 ELSE 0 END) AS  [Under 18],
 SUM(CASE WHEN agegroup BETWEEN 18 AND 24 THEN 1 ELSE 0 END) AS [18-24],
SUM(CASE WHEN agegroup BETWEEN 25 AND 34 THEN 1 ELSE 0 END) AS [25-34]
WHERE id = CUSTOMERS_ID;
END LOOP;
END get_age_group;

it has a very limited success. it only works without the sums and then only if i add birth_date column to the customers table

any help would be very helpfull

(FYI the data aren't real)