I've noticed that tracing is almost universally looked down upon in the online art community. And there is a good reason for that, when you trace you are not producing your own art, you are replicating an image from someone else. But is tracing always bad? Is it always art theft? What about tracing your own work? So many questions.

Personally, I'm an advocate of tracing as an educational tool. I traced video game characters out of instruction books and magazines growing up. At that Age I didn't "know better" but that was a way for me to take that cool character and put it on paper. And I think in doing so I was building motor skills as well as observational skills. Though I did have the wherewithal to claim that "I traced this" and was very proud of it (That picture of Ganondorf had LOTS of lines man!).  But you didn't draw it. Well yes I did, but not really, I traced it. Not all marks we make are art, some are just practice. 

How much different is it for a 7 year old to trace a comic book than for a Art School student to 'study' one of the old masters by producing a copy of their work? You didn't trace it, you didn't make 'Art.' You just copied someone else's work. The key difference being that the Art School student already has the fine montor skills to draw, and the observational skill to  replicate somethning without direct tracing. So to me tracing, especially at a young age, is a developmental tool. And even for new, older artists I recommend to trace things from time to time for understanding.

When does it cross the line? Just like the Art Student's study. When you claim the study or the tracing as your own design, then the study become a fraud, or a forgery if you try to pass it off as the original. Educational uses are fine, but I would keep them in sketchbooks, not portfolios. It's part of how the sausage is made, not the final product.