If they don't know where to stop, then how they can learn where to stop? The only way, I think, is making errors. It's nothing bad in taking a glimpse of something too difficult to understand. Even failed attempts to understand something too difficult is not bad. The only risk is the loss of motivation because of failure.

So generally I would say that they learn where to stop (or never see the depths of the endless pit that is knowledge in the first place) with the help of a good teacher or textbook.

A textbook that tells you just what you need to know, and just enough of what's happening under the covers to let you think you understand, and solidify what you're learning