How old was Jr. F. James Sensenbrenner when he first started as a legislator?
Jr. F. James Sensenbrenner is an official_full_name; How old refers to SUBTRACT(MIN(start), birthday_bio)
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.'