How old is my iPhone?

The XS Max was discontinued in September 2019 and introduced in September 2018.

So, the phone is at least a little over 4 years old and as much as a little over 5 years old.

You may be able to find your purchase date by entering the serial number of the phone in this Apple document: Check Your Service and Support Coverage - Apple Support

Enter your serial number in this link and you'll see the purchase date --> Check Your Service and Support Coverage - Apple Support