Stored procedure "Multiple rows in singleton select" error

I wrote this procedure to look for users within an certein age range,
however, when I run it I get the error message: "Multiple rows in singleto
select".
Why is my select considered a singleton, and what can I do to get multiple
rows back?

(I'm running Firbird 1.0)

SET TERM !! ;
CREATE PROCEDURE SEARCH_USERS
 (MIN_AGE SMALLINT, MAX_AGE SMALLINT)
RETURNS (ID INTEGER)
AS
BEGIN
  SELECT USER_ID
     FROM USERS
     WHERE CAST (EXTRACT (YEAR FROM CAST ('NOW' AS DATE)) AS SMALLINT) -
     CAST (EXTRACT (YEAR FROM DATE_OF_BIRTH) AS SMALLINT)
  BETWEEN :MIN_AGE AND :MAX_AGE
     INTO :ID;
END!!
SET TERM ; !!


Thanks,
Elad