oh boy… that was bound to happen sooner or later to be honest. almost anyone posting art to major platforms will have someone cross post that to e621 eventually and its really up to you if you are ok with that, or want to do something about it

is that legal? can they do that?

thats really much more of a question for a copyright lawyer (which im not), but that depends on the license of the art. unless stated otherwise and assuming that the work is even copyrightable, everything is usually all rights reserved, which (as far as i know) does make redistribution illegal in most jurisdictions. if it was licensed as CC-BY for example, this post should be ok since you are listed as the artist and have links back to your profile and the post itself

but thats the neat part! copyright rarely stops people from distributing art and memes

why is my art there if i don’t even like the 18+ stuff

for all kinds of reasons… for me (when i get bored enough) its always a “the archive must grow” kind of deal. there is this sort of desire to have everything nicely tagged and uploaded in one massive archive. it doesnt really have anything to do with being 18+ or not. there is lots of non-18+ artwork on there as well, but we all know thats not what people think of first. if anything, someone saw your art somewhere and thought it should be added to e6 so others can see it too. i dont think that the uploader had any ill intentions. they probably didnt even think that you might have reservations since youre not listed as DNP

i know there’s a takedown form

nice! have you also seen the DNP list? sending a takedown request will affect one post iirc, getting added to DNP goes a bit farther. getting added to DNP will either deny anyone from uploading your art, or will only allow certain uploads under certain conditions. for example you might be able to request that only original artwork without edits is posted

hope that semi-infodump gives at least some answers :D