how to check the activation date of my iPhone

There is no way to check that. The warranty starts on the date of original sale. You can check the warranty status at: Check Your Service and Support Coverage - Apple Support

Enter this page : My Support - Official Apple Support

Enter this page : My Support - Official Apple Support

Enter page https://support.apple.com/my-support

Then, “Sign in to My Support” with your Apple ID

Activation Date of the Iphone is found

That will not give them the date their iPhone was activated.

It shows the date of purchase, not the date I activated it.