You can do this with a derived table

SELECT
label,
COUNT(*)
FROM (
SELECT
CASE
WHEN Age BETWEEN 1 AND 20 THEN '1-20'
WHEN Age BETWEEN 21 AND 40 THEN '21-40'
WHEN Age BETWEEN 41 AND 60 THEN '41-60'
ELSE '>60'
END AS label
FROM thetable )
GROUP BY label

-Mike