How can I find out when I started using my iPhone 7?

There is no way to tell when you started using it. If you bought it new you can look at the receipt or the charge record and see when you bought it. If it was a new phone number (and not a number moved from another phone you can look at your carrier's billing records and see when you first used that number. And you can call your carrier; they may be able to tell you when you first used the phone on their network.

No way to do so.