That's the rule of thumb we use. Edit: that said, it's telling that Imgur automatically marked a cartoon animated tentacle as NSFW when there's nothing else around it. We're not so silly.
Viewing post in Are tentacles NSFW?
that is a very subjective rule of thumb.
maybe change to, is it forbidden for minors to view that content.
but i am not culturally americanized completely. what would get me in trouble at work is not looking at porn, but looking at it on company equipment or if I do it on my smartphone, looking at it on company time or company network. the very act of it being porn itself is not problematic, but the situations where it would be ok, are hand crafted. most of the time it could be construed as sexual harassment as well, if I show it to others.
so basically, i have no idea what is nsfw. it is a meaningless phrase to me. browsing for apple hardware while working at microsoft might get you in trouble i guess. browsing religious content if your company has policy against relgion at work. and so on.