It is equally hilarious concept...that an adult might think, that as a child they were lucky enough to be born as a blank slate, and hold to a completely neutral position regarding truth claims.
Well...hmmm....every child is born a completely blank slate...they can't even see color, or discern corners, takes a while for the static supplied by the optic nerve to the brain to discern anything...and then only slowly do the lines and colors take on meaning and then depth of field and ears same thing... Parents could tell them the sky is Green or Verde and until they tried to communicate with others they'd believe it.
But then through parenting, indoctrination, brainwashing...whatever you want to call teaching a child to read, write and develop social skills....they become thinking individuals whose thoughts are definitely not in a neutral position, but one that has been formulated by those years of parenting and experience.
We see this clearly with the number of people that stay in their beliefs whether they are raised Muslim, Hindu, Protestant, Catholic, or whatever.... for most that teaching is enough remain in the belief of their parents (unfortunately for racism, nationalism, negativism)...they keep their religion...even if only socially....some may convert to another religion, but most who stray will give up on religion....the majority have those beliefs cemented.
The question being asked here if what if folks were just given instruction on the beliefs of the various religions and it wasn't till they were adults and their minds fully formed prior to making those decisions that are typically made on their behalf by their parents?
As I wrote this, it is my contention that there would surely be even more atheists and agnostics in our world today...