...as long as you are fair and credit the original artist!!
Every artist once was a beginner, never forget it!
And most artists started with copying: from their favorite Mangakas or artists.
I think copying is one of the best ways to learn and to get a feeling for a certain style, because you observe and you reproduce it.
I recently saw someone who copies drawings from other dA artists. Copying itself not stealing. It's not the same like downloading and re-submitting under their names. No, it's drawn by them.
But when I told that person to credit the original artist, he or she just hid my comment.
That's coward, really. There's nothing wrong about copying, but it's wrong to deny it and to claim it as your own work. Then it's plagiarism and that's stealing! And even worse: making profit with it!
I think if you want to learn by copying, do it in your sketchbooks privately. But if you want to upload it, ask the original artists before if possible and credit them and link to the original piece.
No one blames you for practicing, but for lying. And honestly, it's a lie to oneself, too.
Thanks for reading!
Edit: Thank you for so many comments! Please understand that I have to keep the replies short due to lack of time!
I only wanted to point out that copying itself isn't bad. Because many people think it's bad, they are afraid to admit it, and see, it's the famous doom loop.
But the more experienced you are, the more you should replace copying by inspiration. Add more own elements and don't stick too much to the original!
Disclaimer: I'm no jurist and this is my personal opinion only.