One thing about manga, especially shonen, is that it’s far more nuanced in degrees of potentially objectionable material, e.g. violence, nudity, suggestive dialogue, smoking/drinking, than what we normally call cartoons. And they’re geared to multiple age/sophistication levels; not just “kid safe” and “not kid safe”. There are, in fact, a plethora of titles that fall somewhere between those two extremes, and attempting to shoehorn any of them into one or the other is generally a waste of energy (witness how Sailor Moon turned out). (I know it’s the anime adaptation that’s at issue, but it’s surprisingly faithful to the original manga that the point is still valid.)
Take Ranma 1/2. There’s lots of brief nudity, but it never goes beyond breasts (and even that only occasionally), and almost never in a situation where a person wouldn’t normally have his/her clothes off. There’s lots of fighting, complete with walls getting smashed, doors ripped of their hinges, and boulders shattered, but it never results in worse than bruises, scraps, and the occasional comical lump on the head; never blood. This is a modest, baseline level of “not kid safe-ness” (I think the actual term is “young teenager”), just a cut above completely family-friendly. The next step up would be your typical serious martial arts or softcore samurai tale; full-body shots here and there, some blood (mostly little cuts and nosebleeds), an occasional broken toe or wrist. Then comes Jojo’s Bizarre Adventure territory (which includes Naruto). People smoke, drive recklessly, swear, fight, and die. Some have ugly scars, some have ugly lives. But the deaths are still fairly clean for the most part…decapitation, strangulation, stab through the stomach, immolation, sliced in half, etc., and not too graphic. (As a JBA reviewer said, “There’s blood, but no more than you’d expect to find in a human body.”) The highest “normal” level (i.e. something you could find in a Japanese bookstore) is occupied by titles like Fist of the North Star. Here it gets vicious, chaotic, and messy. And offensive as well; child abuse, slavery, tyrrany, rape, and mutilation are common occurrences in these titles. This is the usual “M” rated story in American markets. Finally, there’s the most extreme titles, the rare books you can’t even find at all in most places, one of the most famous examples being Battle Royale. Pools of blood, brains and intestines splattered all over the place, and crimes against humanity. (It’s an amazing story, go check it out sometime.
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My point is, I can see why kids would attempt stuff out of Ranma 1/2, because it looks cool and it never gives the impression that it can cause serious tragedy. I can also understand reenacting, say, a Legend of Success Joe slugfest or a M.U.S.C.L.E. 4-star slobberknocker. Boys do this junk all the time, and if something goes wrong, hey, it’s just a couple of arm bones, it’ll heal. But Naruto…about ninjas, professional killers, who constantly put their lives on the line, and no attempt is made to sugarcoat this…no, it really doesn’t make any sense.