Apple has confirmed the device requirements for iOS 18 and iPadOS 18 for iPhone and iPad.
All iPhone models from the iPhone XS and newer (i.e., those with the A12 Bionic chipset or better) will be compatible with iOS 18.
The full list of iOS 18-compatible iPhones includes all iPhone 11, 12, 13, 14, 15 models as well as... – iPhone XS – iPhone XS Max – iPhone XR – iPhone SE (2020) – iPhone SE (2022)
iPadOS 18-compatible models are grouped into dated categories
– iPad Pro models from 2018 and later (3rd generation onwards) – iPad Air models from 2019 and later (3rd generation onwards)
– iPad models from 2020 and later (5th generation onwards) – iPad mini models from 2019 and later (5th generation onwards)
Support for iPad Pro 10.5-inch (2017), iPad Pro 12.9-inch (2017), and iPad 9.7-inch (2018) will be dropped.