Would the following simple WHERE clause do what's needed? (BTW, it will include those of unspecified gender who joined recently.)
<pre>SELECT shopper_first_name, shopper_surname, shopper_email_address,IFNULL(gender, 'Not known' ) AS 'Gender',STRFTIME('%d-
%m-%Y', date_joined) AS 'date_joined', STRFTIME('%Y',date('now'))-
STRFTIME('%Y',date(date_of_birth)) AS 'Age'
FROM shoppers
<b>WHERE gender = 'F'
OR date_joined >= '2020-01-01'</b>
ORDER BY gender,Age DESC;
</pre>