Lots of positive happy stories so far. I do not disagree with them but want to offer another perspective for balance. 

I think this is why having a good knowledge transfer process and documentation are helpful. You can ramp up someone relatively quick to be functional in some capacity this way. Bigger companies are better at this as they often have more established standard operating procedures, processes, and learning methods stored up. 

Depending on just *how* junior the person is will tell what needs to be done. If they lack such basic skills to grow into being proficient then they should not have been hired and will likely be shown the door soon. That's what school is for. In my experience the problem I see is less about a senior engineer not holding their hand and walking them through everything but rather more a reluctance to learn, try, and persist. Depending on the industry or company culture you find yourself in - if you expect hand holding then you are in for a rude awakening.

Ultimately I think it comes down to the junior person if they will be successful or not.