Does iPhone 15 iOS version depend on manufacturing date?

The preinstalled(not maximum) OS version will depend on how long it's been in storage. If you buy a secondhand unit, it'll have the version the previous owner put on it.

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Where do you plan to buy a "new" iPhone 15?

Apple discontinued most iPhone 15 versions more than a year ago, so if you can really buy a "new" iPhone 15, it has likely been sitting on a shelf in a warehouse for a year or more.....not good for the battery.

Check again regarding a "new" iPhone if you are looking at online offers and look carefully in the fine print.

Apple Inc. no longer sells the iPhone 15. You may have to get it from Apple-authorized resellers and wireless service providers if they still have stock.

All iPhone 15s support iOS 26 - it does not matter when they were made; they are all the same phone and all support the same operating system!

Thanks