What is the average age of patients examined in the laboratory for the October of the year 1991?
average age for first half of 1999 refers to AVG(SUBTRACT('1999', year(Birthday))); October of 1991 refers to Date BETWEEN '1991-10-01' AND '1991-10-30'
SELECT AVG('1999' - STRFTIME('%Y', T2.Birthday)) FROM Laboratory AS T1 INNER JOIN Patient AS T2 ON T1.ID = T2.ID WHERE T1.Date BETWEEN '1991-10-01' AND '1991-10-30'