Been years since I used access (use MSSQL now) but from memory:
SELECT * FROM foo WHERE the_date <= DateAdd("m", -6, now());
You may want to modify this a bit to either include or exclude your endpoints.
I think you only need to wrap literal dates in the # symbols....
Edit: Decided to look up something....maybe want to use date() instead of now() although it should work.
This should help you too.