Calculating Subsets of Data

I have data for records that can span between 1 to 16 months.. I want to be
able to calculate the average activity for the most recent six months in a
query and be able to display that in a report.

I can create the calculation in the query for the entire duration of months
for each record but am stuck on how to have it choose only the most recent
six months of data to do the calculation and understand that the six months
may really only be between 1 to 6 months of data..

Any suggestions would be greatly appreciated..My skill level is somewhere
between meatball surgeon and kinda knowledgeable...