I'm going to make this immediately clear in bold text: I do not think anyone below 21 should be on the internet; let alone any minor to see anything pornographic. I don't think anyone on this site will say otherwise either.
That said everything I found from academic papers, from fields such as social sciences (criminology, psychology and even philosophy), have consistently made a correlated point that whenever porn is made legal: Sexual crimes such as rape drop and whenever it's made illegal sexual crimes such as rape rise. Now of course correlation does not mean causation. It's almost impossible to prove something causes another thing when it's about social behaviors because at the very least you could argue that in all those cases it's just because that culture is different from ours.
Still it is note worthy to say that crime drops whenever porn was made legal, and rose when it was made illegal.
Further the risk might not directly lead to harm of others but in Japan's case people obsessed with anime in general are usually shut-ins that literally do not leave their room for years, this is the opposite problem and this has been such a problem that it has led to birth-rates actually drastically lowering: It should be noted anime isn't the sole blame but it is commonly attributed and shut-ins usually do enjoy anime as a replacement to living.
Further despite what media might present Japan still is incredibly safe compared to places like America and Britain. While you can find individuals that say "this led me to a crime," I can also find people that said "this music led me to a crime." or. "this game made me violent."
Criminals are known to blame everything around them, except themselves.
That said the risk of porn is the risk of porn-addiction, who would've guessed it's the classic: "It's only bad when you become addicted."
When you're younger you're easily imprinted, and this isn't just to point out any applicable the malicious intent but the mere fact your mind is easily imprinted. Note all the people on twitter, bolding announcing their age is below the age of 24, proudly marking themselves with political phrases and acronyms they don't understand. There's a reason the stereotype of modern communists is now kids.
My solution is that I believe parents shouldn't let their kids on the internet at all without supervision, it robs them of experiences in the actual world and poisons their heads with fringe ideas that they don't understand at all. I remember when I was younger the expression: "Ask your parent's permission to go onto the internet." Was common. And every PSA reminded kids that the internet is fun and useful but also dangerous.
Further there was a clear divide between sites:
- "This site is for kids" - Cartoon Network, and Roblox
- Roblox actually made it so you couldn't communicate with anyone if you're not at least 13.
- "This site is for teenagers." - Gaia Online, and Newgrounds.
- "This site is for adults." - General internet forums.
Course nothing stopped a kid from going onto any forums, and it happened a lot, but the division helped separate them. Further your parents could block sites.
Now the problem is that every site is for everyone, and almost everyone has a mobile phone. You can't just block twitter anymore, and twitter literally has everything: Here's a twitter based on LEGO, here's a cute animal feed, and here's the political extremist and the rape fetish porn collection that you can view easily.
So I think the glaring issue isn't loli, isn't weird fetishes, isn't easily accessible porn it's the consequence of everything being so centralized: You take one step to the left and here's the furry pipe-line and you take one step to the right and here's the incel pipe-line.