If you meant it in a "Oh, how were you able to handle the height?" kind of way, that's not really what I meant. It's more the "How did your parents afford that?" or "Why would your parents have been able to afford them?" that is considered rude.

And the reason that it is considered rude is because most questions concerning money can be seen as prying. Most people would not ask someone how much they make unless they have a valid reason. This is true for asking the cost of the shoes as well. If you are having a conversation about purchasing said shoes, then it would not seem rude at all to ask. However, just from her stating that she received nice shoes at 14 doesn't mean that she wants to tell anyone how she got them.

I'm not sure if that really made 100 percent sense, so if you need clarification let me know.