Write SQL query to solve given problem: For how many terms have the oldest current legislator served?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT COUNT(T2.bioguide) FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.birthday_bio = ( SELECT MIN(birthday_bio) FROM current )