Boobs are a definite turn off. Then the features of the face are different and the overall body differences. If I saw a little girl that looked older than her age, say a 10 year old that looked 14. I probably wouldn't be interested. I would be more interested in a 14 year old that looked 10. At least in regards to the physical attraction.

Emotional attraction can happen with a girl at any age, even with women. There just wouldn't be a physical attraction which might make it hard for me to have a romantic relationship with them.