How to enumerate returned rows in SQL?

If it's Oracle, use rownum.

SELECT SOMETABLE.*, ROWNUM RN
FROM SOMETABLE
WHERE SOMETABLE.SOMECOLUMN = :SOMEVALUE
ORDER BY SOMETABLE.SOMEOTHERCOLUMN;

The final answer will entirely depend on what database you're using.

try:

SELECT
    ROW_NUMBER() OVER(ORDER BY age) AS RowNumber
        ,count(*) as usercount 
    from users 
    group by age

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