Raising butt

We have a four month old chin that we adopted a couple weeks ago. We pet her in her cage. She constantly is raising her butt (while she is facing us) to have us scratch her bottom. She often squeaks (not squeals) and will let out a scent. She likes to be scratched all over and is pretty trusting of us to let us touch her anywhere.
Our other chins have never done this, but we rescued them as adults. We weren't sure if it was just simply a matter of her telling us where she likes to be scratched or something else. She doesn't seem particularly submissive otherwise. At what age is she ready to breed? (Not that we want to do that!)
Thanks.