Iphone activation date

Head to https://checkcoverage.apple.com and enter your Serial Number. There you will find the date when you warranty ends or ended. This means that your warranty started a year earlier

Go to Settings->Cellular, then scroll all the way to the bottom. It will tell you when your iPhone was last reset. If you never reset it, then that was the date it was activated.

Unfortunately, you can't. Why do you need it?