They're mangaka, 16 is actually pretty old for them.
Unironically though, Japanese society is kinda ****** in some regards. I dont mean they're inferior, in case I get jumped by nation defenders, I just mean there are some weird things going on, like the work culture, and the attitudes toward age. The shit that dude was doing was legal 3 years before he did it(and was only a 2-year sentence at the time, pretty sure Shonen was considering banning him because he was arrested, not what he was arrested for, because they don't give a shit about morality), and that Rorouni Kenshin guy's stuff was COMMERCIALLY DISTRIBUTED beforehand iirc. It's the reason a bunch of manga have that weird fanservice of underage characters(Pretty sure they did this with Orihime), even if they're great on everything else.
For Oda, at least, I personally look at it as sort of a version of what I call the "George Washington Paradox," where a dude is really good in most regards for the time period and relative aspects of culture, but being in that culture leads you to have been raised in an environment that kind of gives you a set opinion on the morality of certain things, even if those things are really ******' bad.
TLDR: Bad things bad, but have you considered society?
(I still think bad things are bad, and am not condoning anything, even though someone is going to say I am. I'm just saying that sometimes there's a bunch of other things involved, and those things should also be condemned, so it's probably worse than you think.)