You can try the iOS 11 public beta already to see many of the new features and design changes, but you will need to wait until September or later for the official release.
Count on an iOS 11 release date in the fall, right around an iPhone 8 release date. The iOS 11 update will arrive on the iPhone 7 and iPhone 7 Plus as well as older devices, as far back as the iPhone 5s.
We typically see new iPhone-exclusive features, then some features for the iPhone 7 and then more features start to fall off for older phones.
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