How to check user defined schemata creation date in SQL server
You may find the information in sys.traces as long as the information not reset which happens upon service restart. 

If that's not the case, try following query, but this is quick solution, the proper solution in your case is Extended Events which stores the selected trace events information outsize SQL Databases, so that you can retrieve  and monitor such event info whenever you need since the captured data won't be retested upon service restart. 

For more details on all event classes and sys.traces

select PATH, Trace.*
from sys.traces AS T
        CROSS APPLY fn_trace_gettable(T.path, default) as Trace
Where ObjectName like '%YourSchema%'
go