Index, Match, Month before calculating AVERAGE

Hello,

In cell D6 a month can be selected. Based on which month is selected I would like the average to pull through to column AQ. For example,

If Jan-13 is selected in cell D6, I would like the average to be calculated from prior months, up to and including the selected month, e.g.

=AVERAGE(F9, I9, L9, O9) - to include Oct-12, Nov-12, Dec-12, Jan-13 actuals

Or if Jun-13 were selected in cell D6, the average would calculate as

=AVERAGE((F9, I9, L9, O9, R9, U9, X9, AA9, AD9)

I'm not familiar enough to know if an index/match combination will work?

Any help much appreciated

Many thanks, Maddy