My DD's are 3 and 7 months, so I can't speak from personal experience, but I do feel sorry for girls who develop so early.
Though they look and are physically adult, and so society expects them to act like one, but they are really little girls. Generally I imagine they want to be little girls, but are stopped due to their development.
I was a fairly late developer (periods didn't start till almost 16), but I remember a friend's birthday party the summer before yr. 6. We had previously been swimming and my friends mum had spread our cossies out to dry so instead of finding them around the house we went swimming again in our knickers. One girl however already had started developing and I just remember how self conscious and left out she looked as she stayed in her swimsuit, which probably wasn't helped when we then went skinny dipping :o
Personally I think a child is,and should be treated like a child, regardless of physical development. I really hope my DD's are late developers like me. As well as not being able to take teenage tantrums, stuffing my bra never hurt me, and I definitely didn't become 'body conscious' until I was about 13 or 14.
.
Your neighbours DD may be down about her looks, or that she is now looking different to her friends. She could just be going through the awkward teenage hormonal stage, but as no one else is, hers is magnified iyswim.