How to set up child account for “teenager”

It works the same way regardless of age. link-> Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

The only thing the age limits in and of itself, is wether or not "Ask To Buy" can be turned on, but it's not affected until age is above 18.

Start here: https://support.apple.com/en-us/HT208982

This guide should also help: https://support.apple.com/en-us/HT201304