UPDATE TABLE
SET d_date =
(
        SELECT MIN(d_date)
        FROM TABLE
)
WHERE EXISTS
        (
                SELECT FLOOR( MONTHS_BETWEEN (SYSDATE, b_date) / 12 )
                FROM agents
                WHERE FLOOR( MONTHS_BETWEEN (SYSDATE, b_date) / 12 ) >=30
      ) 
AND d_date IS NOT NULL

UPDATE TABLE
SET d_date =
(
        SELECT MIN(d_date)
        FROM TABLE
)
WHERE
        FLOOR( MONTHS_BETWEEN (SYSDATE, b_date) / 12 ) >=30
        AND d_date IS NOT NULL