My Equifax score on MyFICO went down from 816 to 791 when I opened a new credit card account. Then, exactly one calendar month after I opened it, the score went back up to 808. It implies one month of age recovers the majority of the loss from opening the new account.
Average age isn't much of a factor at all, and probably doesn't even make one point of difference, because I have 30+ accounts, mostly around 7 years old or older. The score impact is probably directly from opening a new account and not from any impact on average age.
AAoA wouldn't be at issue as you mentioned, but because it's impossible for AAoA to increase in that one month (unless you had TL(s) drop). And of course that's because AAoA is measured as a whole number only.
IME, I took losses when adding new TLs, but usually would see a gain starting at 2-3 months from the opened date. As we say, YMMV.