how do i find out the age if my iphone 7 plus?

Sold from September 2016 to September 2019, and the highest supported version is iOS 15.

If you want the manufacturing date, there’s a decoder for the fourth and fifth characters of the then-current serial number format included here:

https://forums.macrumors.com/threads/decoding-apple-serials-where-when-hardware-was-assembled-1983-2021-and-apple-model-numbers-1977-present.2310423/

Tap Settings > General > About and locate the Serial Number of the iPhone

On your browser, use this link......Check Your Service and Support Coverage - Apple Support

The warranty coverage has expired, but you will see the purchase date of the iPhone

It will be somewhere between Sept 2016 and Sept 2019