When installing macOS Sequoia 15.3, iOS 18.3, or iPadOS 18.3, Apple Intelligence will be turned on automatically on compatible devices, Apple says

Apple Intelligence has been opt in since it debuted in Oct. 2024, but the next iOS will automatically turn it on. The change is set to rollout with iOS 18.3, iPadOS 18.3, and macOS Sequoia 15.3 Anyone can still turn it off in Settings,

Apple Intelligence will only be enabled by default for hardware that supports it. For the iPhone, that's just the iPhone 15 Pro series, iPhone 16 series, and iPhone 16 Pro series. It goes further back on the iPad and Mac—Apple Intelligence works on any model with an M1 processor or newer.