We have no idea.
What we have found is that the environment of the baby during development, and even the environment of the parent before conception, actually play a role in how the baby turns out.
What appears to be happening is that different levels of certain proteins, and certain RNA strands actually leads to different development, and differential expression of genes.
So, even if you take identical zygotes, and raised them in different women, the development would be different. Also, if you had the same exact DNA, but the egg somehow was formed by different partners, the expression of the genes (even though the DNA is identical) would be different. This is because a man’s sperm and a woman’s egg form in an environment that leaves different concentrations of proteins and RNA and such that will alter how that DNA is expressed.
The question is about a newborn baby from 10,000 years ago. A newborn baby had parents that formed gametes, a union that formed a zygote, and fetal development in an environment that is 10,000 years old.
There is a significant chance that environmental chemical exposure, and proteins/RNA in the parents back then was significantly different than the same people would experience today.
Even if these two children were genetically identical, it’s highly likely that there would be noticeable differences due to these factors. This is referred to as epigenetics.
Epigenetics studies stably heritable traits (or “phenotypes”) that cannot be explained by changes in DNA sequence.
Whether or not this change that would likely occur actually pertains to intelligence or not is anyone’s guess. If I were to strictly speculate, I would guess that today’s humans are slightly more suited for social interaction in densely populated areas, and more suited to picking up new information as quickly as possible.
Given a couple generations, my guess would be that those changes would even out almost completely, given identical DNA to start with.