How long to Octopus cards work?
I know that they deactivate automatically 1000 days after the last reload, but if that happens, you just go to a service counter and when they scan it with an online scanner, it will reactivate, including restoring the old balance.
My question is not about that, but about how long the cards themselves will continue to work. I thought the answer was "indefinitely," but during a recent trip, an interesting thing happened. Among my group, we had a card for each of us left over from a previous trip. One of them had not been used for 10-12 years. It was reactivated without trouble, but each time it was used, the turnstile (or reader on a bus) would make an unusual sound that other cards didn't, even though it otherwise worked. It was not a student, senior, or any other kind of concessionary card. When held against the "check your balance" machine in an MTR station, instead of displaying the balance and transaction history, it just displayed a message in Chinese. Google Translate said that it said "Replace card," but since it was otherwise working, we didn't bother.
So, how long will they continue to work before you must replace them? And, what is causing them to be "too old" such that this special beep is made by the readers? Is it some notion of component aging whereby they recommend replacement after a certain fixed period of time, or is it that there have been security enhancements over time and they want you on a later revision, or something else?
I still have one from the original lot in 1997 that's almost 20 years old now. It would be fun to be able to keep using it.