Welcome, primatologica!

One thing to consider when looking at a young child's level of performance or accomplishments is whether this is coming internally from the child, or the degree to which they may have been coached or pushed in their development.

Here are a few articles which describe the differences well:
http://en.allexperts.com/q/Gifted-Children-3267/Naturally-gifted-hothoused.htm
and
http://giftedkids.about.com/od/nurturinggiftsandtalents/p/how_to_nurture.htm

Regarding the Ruf estimates, it is my understanding that they were developed with the population which was available to study... which translates to a relatively small sample. While it may be a great map of that group of children, it is often wise to be cautious when extrapolating to a larger population. There are several interesting interviews with Dr. Ruf on youtube. One example is:


Regarding developmental milestones, there are several great articles on the Davidson Database... http://www.davidsongifted.org/db/
use the search box to find resources & articles on milestones
For example, here is one article:
http://www.davidsongifted.org/db/Articles_id_10106.aspx

Hope this helps.