How to find when change tracking was enabled?
Run the following query in the context of the database in question in order to get the Change Tracking table names used on the database. (Query via Kendra Little on BrentOzar.com)

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
GO
    select sct1.name as CT_schema,
    sot1.name as CT_table,
    ps1.row_count as CT_rows,
    ps1.reserved_page_count*8./1024. as CT_reserved_MB,
    sct2.name as tracked_schema,
    sot2.name as tracked_name,
    ps2.row_count as tracked_rows,
    ps2.reserved_page_count*8./1024. as tracked_base_table_MB,
    change_tracking_min_valid_version(sot2.object_id) as min_valid_version
FROM sys.internal_tables it
JOIN sys.objects sot1 on it.object_id=sot1.object_id
JOIN sys.schemas AS sct1 on
    sot1.schema_id=sct1.schema_id
JOIN sys.dm_db_partition_stats ps1 on
    it.object_id = ps1. object_id
    and ps1.index_id in (0,1)
LEFT JOIN sys.objects sot2 on it.parent_object_id=sot2.object_id
LEFT JOIN sys.schemas AS sct2 on
    sot2.schema_id=sct2.schema_id
LEFT JOIN sys.dm_db_partition_stats ps2 on
    sot2.object_id = ps2. object_id
    and ps2.index_id in (0,1)
WHERE it.internal_type IN (209, 210);
GO

Find the create_date for the change_tracking table(s) found in the first result set by querying sys.internal_tables in the context of the change tracking database.

SELECT * FROM sys.internal_tables;