I guess so. Still that would make peoples brain chemistry different in a sense? Since the type-specific brain patterns probably were set in stone at a very young age.
Again I disagree, but I am lacking in formal evidence (again).
I believe that responses are not "set-in-stone" at least until an individual reaches the
"formal operational" stage of Piagetian development (
i.e. age 10-12). It's basically accepted that children have no./weak concept of the abstract, absurd, or hypothetical until they are at least this developed. Probably because there are many tracts and connections yet to be made? (this might be one of those fateful moments where I am trying to convince myself as much as yourself)
Children are generally more plastic and malleable than adults in almost every regard.
However, yes, "it would make people's brain chemistry different in a sense" that certain groups of synapses are more active over a lifetime. The chemistry you refer to is basically the release and re-uptake of neurotransmitters, the most popular of which have already been named in this thread.