Oracle: how to "group by" over a range?

SELECT CASE 
         WHEN age <= 10 THEN '1-10' 
         WHEN age <= 20 THEN '11-20' 
         ELSE '21+' 
       END AS age, 
       COUNT(*) AS n
FROM age
GROUP BY CASE 
           WHEN age <= 10 THEN '1-10' 
           WHEN age <= 20 THEN '11-20' 
           ELSE '21+' 
         END

Try:

select to_char(floor(age/10) * 10) || '-' 
|| to_char(ceil(age/10) * 10 - 1)) as age, 
count(*) as n from tbl group by floor(age/10);