Perhaps more than any other, YA is the one category where word count is very flexible.
For starters, 55,000 – 69,999 is a great range.
The word round the agent blogosphere is that these books tend to be trending longer, saying that you can top in the 80Ks. However, this progression is still in motion and, personally, I’m not sure about this. I would say you’re playing with fire the higher you go. When it gets into the 80s, you may be all right—but you have to have a reason for going that high. Again, higher word counts usually mean that the writer does not know how to edit themselves.
A good reason to have a longer YA novel that tops out at the high end of the scale is if it’s science fiction or fantasy. Once again, these categories are expected to be a little longer because of the world-building.
Concerning the low end, below 55K could be all right but I wouldn’t drop much below about 47K.