As it continues the 64-bit app transition, Apple will begin alerting customers running macOS High Sierra 10.13.4 when they are using an outdated, 32-bit app. The alert will only happen when you open a 32-bit app, and it will happen once, Apple told me earlier today.
The alert is a way of communicating to customers that some action needs to be taken, albeit not immediately. Apple said they don’t have a firm timeline of when 32-bit apps will stop working, but they informed developers at last year’s WWDC that macOS High Sierra would be the final version to run 32-bit apps without compromise.
The alert that is shown when you open the app tells people that the developer needs to update the app to make it compatible going forward. You can still use the app in its current form, but at some point in the future, it won’t work.

Apple is taking this step to ensure that the Mac apps we use are as advanced as the Mac itself.
Apple has posted a support article explaining more on the transition to 64-bit, as well as offering a few answers for affected users.
There is no need to panic if you see this alert, but please do take note of it. You should contact the developer of the app to see if they plan to update it and when.
What does this mean for Civilization IV? And Age of Empires III?
Disaster…
Age of Empires III is now 13 years old. At what point will Apple have your permission to move the OS forward and leave apps more than a decade old behind?
This is a terrible warning message. It suggests to users there is something wrong with the app now, as you launch it, rather than requiring improved compatibility for an upcoming OS update.
Sure, it’s no sweat to more advanced users, but it’ll send countless Moms and Pops to their phones to ask the kids what’s wrong with their app.
I’m a little confused…I was unaware Apple needed my permission. I’m also confused as to how one could infer that from my comment.
But glad to know they do–permission not granted!
I’ll rephrase. At what point may they move the OS forward without you referencing such a move as “a disaster”?
Why would I not deem it a disaster to be unable to play two of the best games ever created (yes, that’s a subjective judgement)?
There’s tradeoffs with everything in life.
Easy: emulation. Like all the other great games from decades past.
I suppose for the same reason I didn’t think it was a disaster for Sony to move to the PS3 and leave behind some of the greatest games ever produced. Technology, like life, moves on and good games get left behind. This is not, in anyway, a disaster.
If those games are some you still play with regularity, then keep an older MacBook or iMac handy running an earlier version of macOS/OS X to fire them up when you get the itch.
Also, allow me to apologize if I was a bit harsh in my original reply. I mapped on to you the frustration I get when Apple makes a move like this, with a great deal of advance warning, and message boards are slammed with users thrashing Apple for ruining their lives.
True, but there is no Mac on which this warning is not true: 32-bit apps are not optimized to run on your High Sierra-running Mac. And it should be updated to improve compatibility with the architecture.
That it’ll stop working in a future version isn’t even mentioned, and I think that’s a problem.
…I can’t tell if I’m agreeing with you, either.