They've never been out of the "atheism category". "As far back as 1772,
Baron d'Holbach said that "All children are born Atheists; they have no idea of God."
https://en.wikipedia.org/wiki/Implicit_and_explicit_atheism
From Wikipedia on "implicit atheism"
Smith defines "implicit atheism" as "the absence of theistic belief without a conscious rejection of it". "Absence of theistic belief" encompasses all forms of non-belief in deities. This would categorize as implicit atheists those adults who have never heard of the concept of deities, and those adults who have not given the idea any real consideration. Also included are agnostics who assert they do not believe in any deities (even if they claim not to be atheists). Children are also included, though, depending on the author, it may or may not also include newborn babies. As far back as 1772,
Baron d'Holbach said that "All children are born Atheists; they have no idea of God."
[2] Smith is silent on newborn children, but clearly identifies as atheists some children who are unaware of any concept of any deity:
Before I dissect, I fully expect someone to come along and suggest Wikipedia is hardly a source for understanding these things.
I first wish to dispute the assertion: "Absence of theistic belief" encompasses all forms of non-belief in deities. I don't think that is possible. That absence encompasses all forms. Just the way it is phrased is questionable, but as I get what it is trying to assert, I'll overcome the doubts about that. If I lack belief in Thor as Creator God, and assert that, for me, Thor is a mythological being - that is not (remotely) absence of belief. While it is plausibly absence of specific theistic belief in Thor as God, it does not encompass, for me, non-belief in all deities.
Because babies are, from our understanding of babies, unable to communicate beliefs (in anything), then I do see it as fair to suggest they have absence of theist belief that does encompass all forms of non-belief in deities. They would be unique in all of atheism. An adult atheist is highly likely (I have never heard of an exception) familiar with conceptions of at least one deity. Whereas a baby is understood as having zero conceptions of any deity (or anything at all). Thus, for the adult atheist, it would be very challenging (I'd argue not possible) to maintain absence of beliefs that encompass all forms of non-belief in deities. For once the concept is introduced the level of belief-acceptance begins. Introduce same thing to a baby, who is understood to be incapable of belief-acceptance, incapable of conception, and the absence of belief is able to be maintained.
So, I reject the assertion: "This would categorize as implicit atheists those adults who have never heard of the concept of deities, and those adults who have not given the idea any real consideration."
I reject the idea of any adult who has never heard of the concept of deities. If it is a specific deity, and only that one up for discussion, I think the point has some merit, but not equivalent to absence as a wholly encompassing position. Such that if I run into an adult who has never heard of Flying Spaghetti Monster, they would be (prior to meeting me), absence of belief in FSM. Once we meet, and I introduce the concept of FSM, they are no longer absent of the belief. I realize that may have counterpoints, and I feel prepared to have that discussion if and when it arises. But wish to be clear that for the baby, they are understood as incapable of conceiving of FSM after I have verbally explained it to the infant.
I am therefore rejecting this questionable notion of "adults who have not given the idea any real consideration." Rejecting it based on logic. For it to be logically consistent with absence, it would be ANY consideration. The real part would be confusing. Who determines that? How? For all we know, babies are experiencing real consideration when the concepts are communicated to them from .... wherever.