SELECT `friends`.`friend_age`
FROM `friends`
WHERE `id_user`
IN(SELECT `users`.`uid` FROM `users` WHERE `age`=13 AND `number_of_friends` <> 0)
AND `friend_age` <> 0 ORDER BY `friend_age`

select distinct f.friend_age
from fiends f
  join users u on f.user_id = u.uid
where
  u.age = 13
  and f.friend_age<>0
order by f.friend_age

select f.friend_age, count(*) cnt
from fiends f
  join users u on f.user_id = u.uid
where
  u.age = 13
  and f.friend_age<>0
group by f.friend_age