Among the current legislators who have served for more than 6 terms, how many of them were born after 1960?
served for more than 6 terms refers to COUNT(bioguide > 6); born after 1960 refers to birthday_bio > = '1960-01-01'
SELECT COUNT(CID) FROM ( SELECT T1.bioguide_id AS CID FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.birthday_bio >= '1960-01-01' GROUP BY T2.bioguide HAVING COUNT(T2.bioguide) > 6 )