My recent app spree lowered my AAoA on EQ from 10.1 to 9.9. Since 1 month doesnt really equate to 0.1 (12 months but AAoA measured in tenths of a year), would the start of November bring my AAoA back to 10.0 years? I have 30 accounts with 9.9 AAoA right now. (the underlying question is whether I should expect a score boost when my AAoA goes back over 10 years? I cant tell if I had score hit for it going below as I had high utilization report at same time due to hurricane storm damage repair.)
With that many accounts in the equation my bet would be that you would return to 10 with next months roll around. The AAoA does not appear to be dependent on the accounts reporting for the month but rather the passage of time and the age of the specific accounts. I don't believe we have ever established a proven number of years where there is a point boost for age. I believe that I have read anecdotally that more than 3 and more than 5 (or 6?) seems to provide a boost to some members. I don't recall reading anything about 10 years being a significant factor but YMMV.
In credit reporting there is often such a variety of factors in play (utilization, inquiries, new accounts aging, etc.) that it is difficult to attribute a score change to one factor. The only one I know for sure is a brand new delinquency will tank you and I'm not eager to find out by how much.
Assuming you are at exactly 9.9 years (i.e., there is no rounding going on there), one more month won't take you over. You have 297 years of total history, you'll get another 2.5 years with one month across 30 accounts, bringing you to 299.5. 299.5/30 = 9.98. If you're actually slightly above 9.9, you'll probably cross the 10 year mark.
Just do the math if you want specifics...7.8 yrs for example would be 7 years plus X months. 0.8 = X months / 12 months = 9.6 months. So, 7 years 9 months.
BTW, cross-reference your EQ report from Equifax.com with your myFICO report from time to time. A couple of times I found that Equifax.com's AAoA to be inaccurate.
ETA....YMMV on the score change. On November 1 your AAoA will hit 10 years (as seen by FICO). You might see an increase due to the aging of your new TL(s), but I don't think your AAoA alone will improve your score because you have new accounts reporting.