Will IPadOS 18 be compatible to IPad Air 2?

No. Your iPad Air 2 is capable of update to iPadOS 15.8.x (currently iPadOS 15.8.2). The Air2 fundamentally does not support iPadOS16 or later

You can find a list of devices that will be supported by iPadOS 18 towards the end of Apple's iPadOS 18 preview documentation:

iPadOS 18 Preview - Apple

As you will see, iPadOS18 will be compatible with these models of iPad:

• iPad Pro (M4)
• iPad Pro 12.9-inch (3rd generation and later)
• iPad Pro 11-inch (1st generation and later)
• iPad Air (M2)
• iPad Air (3rd generation and later)
• iPad (7th generation and later)
• iPad mini (5th generation and later)

No. An unit too old for 16 or 17 will be too old for any version higher than those.

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