AAoA isn't calculated on a specific year (e.g. 2011, 2012, etc.) or specific month (December, January, June, etc.). The age is calculated starting from the month the account was opened and your CR will list the open date as always on the first of that month. So, a new account opened anytime within December will technically be 2 months old on January 1. Those two months are December and January. If you ever look at your FICO report the day after your new TL reports, it'll always say (depending on your scoring bucket) that your newest account is 1 month old.
As it applies to AAoA, the age of a TL is calculated by figuring out how many months an OC account had been opened. Again, any TL opened this month is 1 month old. AAoA doesn't care which month it was opened. So, this month next year that TL will be 13 months old, then 25 months after that and 37 after that.
In terms of rounding down, that is done after the average had been created. You don't round down before you create that average.
Let's say you have a CC that was opened in October (10/2011), a charge-off loan that was initially opened 5/2004, and a closed CC opened in 4/2001. The ages of these accounts respecifvely are 3 months, 92 months, 129 months. The average of 3, 92, and 129 is 71.6. 74.6 divided by 12 months is 6.21 or roughly a 6 year and 3 months AAoA. FICO then rounds down and you'd have a 6 year AAoA reporting. By next August, in this example, your AAoA turns 7 years (assuming nothing is added or removed).
