It is my understanding, that the length of time is calculated whether or not it is open or closed for 10 years. The exception, of course, being when you leave it open, in which case, it will show up for over ten years. I see this on my own credit report, since I have three accounts open for 10 years, and 7 accounts opened in the last 3 months. Despite this fact, all agencies calculate my average account opening at over 6 years because I have 30+ closed accounts still on my report.
The other good thing to remember is that if your longest Card history is with Amex, you can always open a new account with them, and close the old one, and the new card will report to the credit bureau being opened at the time you opened your first account with them.
I guess my thought would be if length of time is your concern, which does affect your score, I would not close down my oldest account unless it was Amex. And yes, the closed account does add to credit age, up to ten years. And yes, it keeps aging up to those 10 years.