I did teach Python at a computer science school (people there already had 2 years of scientific studies after "bac"), and I was amazed to see how hard it was for some of them to understand that in Python :

>>> 4+2
6
>>> "4"+"2"
'42'

So yes, I guess the key is about understanding what types are. The same kind of issues arise between using a variable and the variable name.

Now, I'm not sure how much this is teachable and when (ie, maybe it's a kind of skill you've to learn when you're young to really grasp). I started programming when I was 11, so there may be something around it, but I don't have much data on that.