As a Library Studies student and someone who has just started working in the library system (weird kind of mid-life crisis huh?) not to mention as the parent of a 12 year old, this topic is of great interest.
A think one of the central issues might be what is a 'Young Adult'? The American Library Association defines this group as 12 to 18 year olds. Which to my mind is a bit misleading as I think of young adults
as 18 to 20 ish. On top of that, the range of 12 to 18 is a pretty broad one in itself in terms of development and interest. I personally feel that what is appropriate for a 16 to 18 year old is different to what is appropriate for a 12 to 13 year old, but I'm a bit old fashioned.
I was happy to let my daughter read the first two Tiffany books when she was eleven, but I held off on I Shall Wear Midnight. I'll relent next year when she hits high school...and it won't be long before she asserts her own choices anyway! I can only guide from here on in I suspect. But I think it is as role of parents to take an interest in what our kids read, not for libraries to censor. That ends up going down a very interesting path and it is worth looking at what is
challenged for kids to read...not always what you might think http://www.ala.org/advocacy/banned/frequentlychallenged/challengedbydecade/2000_2009
As for what makes the Tiffany books 'YA' I think that they do, more than the other Discworld books focus on an adolescent’s perspective. For that reason I think they are a great example of YA writing and not half as confronting as some other offerings in the category. I also think good YA writing should be enjoyable to an adult too, just voiced from a different perspective.
Edited to say that my daughter also tried a couple of the the other Discworld books when she was about 11. She just didn't 'get' them in the same way as Tiffany.