Pardon my ignorance but I really don't know why kids should not be allowed to watch porn. I tried to look it up, but all results I looked at either claim it is ok for children to watch porn or just jump to teach you how to prevent your children from watching porn while assuming you already know why porn is harmful. Honestly, I doubt that there will be significant damage, especially if the people watching are already 16 or 17 years old.

However, that is not my expertise, so I can't say for sure that it is completely ok for teenager to watch porn.

Nevertheless, for the sake of this conversation, let's say, it is indeed really harmful for underage people to watch porn. Then, why don't Google or Apple just make a toggle? Or, since you already need an account to use Google Play or iOS App Store, just filter out all the porn apps when the users are deemed to be too young to properly enjoy porn. The fact that it is dangerous for a 6-year-old to drive a car does not mean cars should be banned for the entire humanity.

On the other hand, if watching porn for young people is indeed that harmful (enough for justifying banning them altogether on the largest 2 mobile app distribution platform), the future of humanity is already doomed, because they can already open up Google and search "porn" and start watching them.