How do I change the age on iPhone

Devices have no age setting.

Apple ID's have an age based on the birth date provided when creating them. This age can be changed at any time to a different one except if it's set to below 13. link-> Update the birth date associated with your Apple ID - Apple Support

If your child is using your Apple ID, then the age is your age, and changing it would affect your Apple ID not just one device. Though the age setting does not affect anything directly except the Ask To Buy feature in Family Sharing. Link-> Approve what kids buy with Ask to Buy - Apple Support

If you need to set the age to your child's age for some reason, they must be using their own Apple ID. link-> Create an Apple ID for your child - Apple Support

Do note, no settings on a device are age dependent. You can restrict any settings and limit the age gap of apps and movies and Tv shows that can be downloaded from the stores in Settings->Screen Time regardless of the age set for the Apple ID. Link-> Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

How old is your child? If they were under 13, did you create an Apple ID for them? Are they a part of an Apple Family Sharing plan? ... and now, they are older and you want to convert that ID to an "adult" one? ... or they are still under 13 and you just want to change their age?