Write SQL query to solve given problem: How old was Jr. F. James Sensenbrenner when he first started as a legislator?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(MIN(T2.start) - T1.birthday_bio AS DATE) AS AGE FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.official_full_name = 'F. James Sensenbrenner, Jr.'