Any teenager looking for porn find a way to gets porn. Nothing can stop them.
Are you not capable of imaging how ease of access and availability increases use?
What's easier: opening the browser and searching for porn, or installing a third party app marketplace, going into the settings to enable it, finding a .json document for a source that has a porn app and adding it to the app marketplace, then downloading the porn app?
on my little kids devices, safari is blocked with built in parental controls. this isn't difficult
Built-in parental controls can also block kids from installing apps from alternative app stores.