For the patient who got the laboratory test of uric acid level as 8.4 on 1991-10-21, how old was he/she at that time?
how old at that time refers to SUBTRACT('1992', year(Birthday)); uric acid level as 8.4 refers to UA = '8.4'; 1991/10/21 refers to Date = '1991-10-21'
SELECT STRFTIME('%Y', T2.Date) - STRFTIME('%Y', T1.Birthday) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.UA = 8.4 AND T2.Date = '1991-10-21'