Database: thrombosis_prediction
Question: What is the average age of patients as of year 1999 examined in the laboratory for the October of the year 1991?
SQL: 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'