Great question.
On the other hand, let's explore your example a bit further. Suppose you had one credit card that was 7 years old (and no other accounts). By then opening a ton of credit cards, that causes your AAoA to drop precipitously. (From 7 year to 0.87 years) The existing system permits the scoring system to not only capture that drop but to keep track of that. In the "open accounts only" approach, however. a person could artificially erase that scoring penalty by closing a lot of the open accounts.
Indeed, we often run into people who are newcomers here who think that if they pay off a card and close it, then that should cause everything bad about the account to vanish from scoring impact, including derogs, AAoA, etc. The current system ensures that once you make a decision that hurts your score, there's no immediate action you can take to make it magically go away, with the exception of utilization -- and even there the advent of trended data may change that too. (TD analyses, if they comes to maturity, will be able to examine a persons CC balances and payments month by month for the last 24 months, rathen than just the most recent balance.)
In the "closed and open both count" approach to AAoA, the closed accounts will eventually fall off, so the worry you have about accounts that have been closed for 8-9 years helping a person's AAoA will get resolved eventually. The closed and open approach also protects a person who has had a 20 year old credit card that gets inadvertently closed. Such a closure does not have any short or medium term affect on his age -- but in the open only approach it might cause his Age of Oldest Account to go from 20 to 2.
Note that scoring systems can still track open accounts separately if they want. FICO indeed does track how many open credit cards you have -- one open card helps you far less than three, for example.
A scoring system could in principle count how many months per credit card you have had it open (and therefore how many months you have had an opportunity to miss a payment). As an example, if you had three cards each open 12, 24, and 36 months respectively, that would be 72 possible months. A scoring system could certainly track that if it wanted to. That approach is just as easy to deliver dishonest results with as any other -- easier perhaps. All you'd need to do is open a bunch of junk cards (store cards typically) in the first few months of your profile and then not close them. After a few years you'd have racked up dozens and dozens and dozens of OK's where you hadn't missed a payment -- simply at leaving them at $0.
