If my parents had the feature to be alerted about porn their kid’s device while I was a teen they would have sent me to a conversion camp, and that is not an exaggeration.
Apple thinks the appropriate time for queer kids to find themselves is after they turn 18.
If you're just downloading and looking at porn, no problem. It only becomes an issue if you're sharing porn via Messages or storing it in iCloud. And to be fair, I don't think they're alerted to the nature of the pornography, so you might be able to avoid being outed even if you're sharing porn (or having porn shared with you).
Edit: I'm wrong in one respect: if the kid under 13 chooses to send a message with an explicit image despite being warned via notification, the image will be saved to a parental controls section. This won't happen for children >= 13.
Apple thinks the appropriate time for queer kids to find themselves is after they turn 18.