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

"For users new or upgrading to iOS 18.3, Apple Intelligence will be enabled automatically during iPhone onboarding. Users will have access to Apple Intelligence features after setting up their devices," the notes for the iOS/iPadOS release candidate to developers reads, spotted by MacRumors.

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.