My daughter, who is 5, has already said to me, "Mommy when I grow up, I want a Louis Vuitton bag just like you".
The key word here is "grow up". When I feel she COMPLETLY UNDERSTANDS the care, and appreciation you must give to something that costs so much, then and only then will I get her one.
So, does your daughter at 12, completely understand this? Does she appreciate that you'd be getting her a very expensive (for a 12 year old) bag? Or, does she know you can afford it and just wants it, and figures that because you can afford it you'll just get it?
If the latter is true, then you'd only be doing her harm by getting it. If the former is true, then she'll be a very happy young lady.