In calculating AAoA, FICO considers all OC accounts (trade lines), open or closed, that are still included in your CR. Add up all the months since their date of opening, and divide it by the number of accounts. That is your AAoA.
AAoA is only part of your length of credit history scored by FICO, FICO also looks at the date of your oldest reporting account.
The important thing to remember about AAoA is that the FCRA does not mandate how long the CRA must continue to include old, closed accounts in their credit reports. They get, literally, millions of reports daily, and devote umongous amounts of storage to maintaining all this data. It is up to them when, and if,they purge old, closed accounts from your CR. They will usually maitain the account for ten years from closing, for that is the date of expiration of most prior dergogatory reporting that they can make under FCRA 605.
So it is possible that you can have an old, closed account with a 15-20 year history suddenly disappear from your CR, and lower your AAoA. That is why the saged advice is generally not to close old accounts.
