How do I see when an apple device was first registered to my Apple ID?
How do I see when my apple device was first registered to my Apple ID/ first signed in?
iPad Pro 9.7-inch WiFi, iOS 9.3.2, null
You can't.
If the device is registered to the device you can see when it became registered, but that only happens if you do particular things with the account on the device : View and remove associated devices in iTunes - Apple Support