The age of that account would be 4 years. It starts from when it was opened and continues to increase until it falls off your record, which is ~10 years after the account was closed. So that account when it drops off would have an age of ~14 years.
CK only calculates based on open accounts, FICO does both open and closed accounts. That's the source of your discrepancy though I didn't calculate it out by hand in your case.
THANKS! This is super helpful. I was wondering why my CK AAoA was so low.