To find out the Earliest Date when a Field was being tracked. Use the Inspector and SOQL to query against the [ObjectHistory] Object. While you could possibly do this in a report, your Org will may have many records (In this Example Leads) and therefore it will take some time to update the report to query.
Therefore, using the Lead Object as an Example, to find the earliest date when LeadSource was being tracked in the LeadHistory object, you would need to sort the results by the CreatedDate field in ascending order and then limit the results to 1 record.
Note: You would use the following SQL Query from a tool such as the DataLoader or the Inspector.
Here at Swantide, we recommend the Inspector.
SELECT Id, Field, CreatedDate FROM LeadHistory WHERE Field = 'LeadSource' ORDER BY CreatedDate ASC LIMIT 1
You can repeat this process for other Objects that allow for Field History Tracking.