I'm speaking more to the viewers than the artists themselves.
If an artist does not specifically state that they want criticism, do not give criticism. That's it. The fact that strangers feel so entitled to offer their voice when it is not asked for is baffling.
Personally, if I want criticism, I go to people who I know are educated artists, whose opinions I can trust.
That being said, artists should have times where they do ask for criticisms and seek to educate themselves, especially when they hit roadblocks within their art. But forums and videos where they relentlessly pick apart young artists' work under the guise of "I'm just trying to help you improve" is not helpful, its bullying.
"Artists need to keep their works to themselves", absolutely not. Sharing and creating art is a form of bonding and create communities with common interests. For both serious artists or those who just draw for a hobby, for fun, sharing with their peers is an exciting part of the process.
When I was young, I created tons of OC's and fan art to spread my love for various media.I loved to draw, I did it every single day since before I can ever remember. Honestly, in elementary school I was known as the girl who drew everyone's favorite Pokemon. Passion does not die out with age. I draw original characters, scenes, I like to experiment. Even now, I'll create illustrations without first creating thumbnails, or researching color palettes. Maybe the anatomy was wonky but I liked the flow. I create art for me, and to share it means I am sharing a privilege viewers take for granted. I take my art education very seriously, and you'll know when I'm studying, because that is when I ask for criticism. Everything else? I drew it for my own reason. Personally, criticism doesn't bother me, but I've seen teenagers, even preteens being subject to "bullying" because someone wants to show off how much more they know. This is how passion dies. Criticism should never be only negative. As someone who intends to educate, you must point out the aspects where they were successful, and where they need to improve. A criticism should inspire the artist to move forward, not back down.
Let people draw. Not everything has to be a lesson, and not everyone's critique is wanted, well thought out, or educated. I'd always take my professors word before I'd ever consider going to deviantart.