when was patient 13841 prescribed a medication until 09/2104 for the first time?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 13841 ) and strftime('%y-%m',prescriptions.startdate) <= '2104-09' order by prescriptions.startdate asc limit 1