The date of foreclosure wouldn't enter into AAoA. Whether the foreclosure was 6 yrs ago or yesterday, the open date on the mortgage is the same.
OP, the mortgage is over 7 years old if the open date is 6/2004. If this account drops at 7 yrs from DOFD, or 7-7.5 years minus this coming June if so reported on TU, then there will be some sort of impact to your AAoA which can raise or lower your score on the history alone.
OP, you'd need to calculate AAoA. AAoA is the average age of ALL of your OC accounts, whether they are opened or closed, good or bad. There isn't enough info posted to determine AAoA. You'd calculate how many months each OC account has been opened through the end of this month. You'd then average the sum of those OC accounts and then divide 12 months into that to come up with an AAoA measured in years. FICO rounds down to the nearest whole number in calculating AAoA. To cheat, your FICO report might show this info on pages 2 or 3 of your FICO report. Other CMSs like CreditKarma or Equifax.com do show AAoA but they calculate it incorrectly and should be ignored.
You'd want to compare the 7.5-8 yr age of the mortgage in relation to AAoA. As mentioned, if the age of the mortgage is lower than your AAoA, then you could see a gain on that alone as AAoA could increase. Conversely, if the mortgage is one of your oldest accounts, and your AAoA is younger than 7.5-8 yrs, then you could actually see a decrease if the mortgage is removed. Other factors come into play outside of the history like mix, how the foreclosure reports, your scoring bucket, overall credit, etc.
Like the prev. poster, I think you'll see a net gain. It isn't your oldest account. You won't be rebucketed because the BK reports. I vote for a net gain.
