I can't restore my iPhone's backup because it's iOS is too old

Update your iPhone SE 3rd gen to the latest supported iOS version (17 or later), then retry to restore data from the backup

Rule: you cannot restore data from a backup created with an operating system version more recent than the one currently running on the destination device. That's true for all computers or smartphones, regardless of the type of operating system you are using (Windows, macOS, Android, iOS, iPadOS, etc).

If the destination device cannot be updated to an iOS version ≥ than the one you used when creating that backup, you won't be able to restore any data (except data synced to iCloud).

If your iPhone lost its functionality after an update procedure, please describe your problems and we'll try to help you, or contact Apple Support directly using this link:--> Contact Apple for support and service , or have your device inspected at an Apple Store or Apple Authorized Service Provider.

Regards

Giulio

Stating the obvious. That is a one dimensional answer. What if the phone is not upgradable? Or the upgrade will cause loss of functionality, as it did to my iPhone? American creativity appears to be on a decline.

This will most likely fix the issue.

The phone can run iOS 17 so that would enable cross functionality between devices.

there is also a security fix in the latest iOS which should be installed.

But the phone is upgradable to iOS 17 soooooo no idea what you mean