when was patient 73423 for the first time prescribed a drug via po/ng route until 63 months ago?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 73423 ) and prescriptions.route = 'po/ng' and datetime(prescriptions.startdate) <= datetime(current_time,'-63 month') order by prescriptions.startdate asc limit 1