Hi a212830,
from the docs http://docs.splunk.com/Documentation/Splunk/6.2.0/SearchReference/Metadata
firstTime is the timestamp for the first time that the indexer saw an event from this host.
Does this not do the job for you? For me it works:
The metadata command doesn't provide you mapping of index/sourcetype and host at once. Try following (will take more time but may be the only option)
| metasearch index=* sourcetype=* host=* | stats min(_time) as firstTime by index sourcetype host
Sorry - actually needs to include host, index and sourcetype. I get a lot of data for these hosts, so using metadata would be a hug help. We are concerned that data is dropping.