An account that is closed will age for 10 years past the closure, helping your average age, but at 10 years it drops off your report. At that point it can lower your average age if you have newer accounts on your account that now have more of a say in your average age. If you kept the same revolvers and rarely opened new accounts your average age would keep increasing and your score potential would as well.
Many people like to keep cards active by using them once every 3 months or by automatically charging a small recurring bill to them. They then often pay the bill in full before the statement cuts, to prevent the card from showing a balance.