On my CR, I have 3 open accounts and 1 closed account with an AAoA of 8 years. I wish to know the effect of a 4-card app spree on my AAoA. This is how I did it using Excel.
The first 4 rows represent the current cards on my CR.
The last 4 rows represent my new cards.
The entire column C is formated as mmm-yyy.
Cell C1 labeled "Now" represents today's date. The formula used is =TODAY() but you can easily change this to any date.
To calculate the "MTHS" column, I used the formula for the Round Up method described in the reference listed below.
To calculate AAoA, you take the average of the rows of interest and divide by 12 to convert to years. See the example in the formula bar.
The first 2 AAoA calculations was to convince myself that Closed accounts are included in FICO's calculation of AAoA. Since ScoreWatch gave my AAoA as 8 years, I conclude that yes, Closed accounts are included in the AAoA.
The last AAoA calculation (7 open + 1 closed) shows the effect my 4-card app spree had on my AAoA, which is now 4 years. I basically halved my AAoA. 
The 3rd AAoA calculation shows what happens when the Closed account eventually drops off the CR.
I know myFICO users can be a little fantical when it comes to things like this. Hopefully, my explanation of how to use Excel to calculate AAoA will be useful to someone.
Reference: How to calculate the number of months between two dates using Excel
EDITED: To simplify AAoA calculation using AVERAGE function.
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