https://wccftech.com/airtag-requires-ios-14-5-and-ipados-14-5-or-above-to-function-wont-work-on-older-devices/

According to Apple, the AirTag will only work if you have iOS 14.5 and iPadOS 14.5 or above installed on your iPhone and iPad.

We are slowly and gradually unearthing a lot of details regarding AirTag and the latest one should not come as a surprise. According to Apple’s very-own tech specs page for AirTag, the tracker will only work if you have iOS 14.5 and iPadOS 14.5 installed on your iPhone and iPad. The update is set for release next week.