So you're wanting to know some basic dates around a schema in SQL Server. Unfortunately the "sys.schemas" view doesn't have anything to indicate create or modify dates. Furthermore if the schema creation isn't logged, then the only way to review this is to relate the schema to the "sys.objects" view.
Here is the query to looks at this:
count(so.object_id) as ObjectCount,
min(so.create_date) as EarliestCreateDate,
max(so.create_date) as LatestCreateDate,
min(so.modify_date) as EarliestModifyDate,
max(so.modify_date) as LatestModifyDate
sys.objects so on so.schema_id = ss.schema_id