That they CHOOSE to adopt ... because they can choose to reject it. And many do.
No, children do not have the cognitive development to choose what they are exposed to as they grow up. A five year old in Sunday school isn't choosing to be there. Children in religious households have no choice to avoid church. It might be boring but that is why there is Sunday school for the young, as it offers more entertaining religious lessons. By the time a child begins to have autonomy they have been well indoctrinated in belief. This is addition to the biological nature of religious belief.
Since the more publicized alternative to religious belief that has come from Dawkins, Hitchens, et.al. there have been more people realizing it is an alternative for them if they prefer to reject religious belief. As we see many more religious people see atheism as a threat. Any parent will want to guide their children to be a believer, just like them, versus allowing them to be exposed to the option of non-belief. In this thread we can see Tony's hostility to atheism, along with yours. This is mostly observed in the misrepresentation of non-theism.
How consciously or "intellectually" they debate this choice within themselves is also their choice, and their responsibility. The key, however. remains that it is a choice.
The human brain doesn't fully develop until about 27, way after they have been exposed to religious concepts.
Religions don't make anyone believe anything. Neither does social or political propaganda. All any of these forces can ever do is massage and support ideals and biases that we already hold. They cannot make us believe anything.
You need to study how strong an influence propaganda can have on minds. It is why trump and other corrupt politicians have gotten so much public support. These voters are not objective and making sound choices. The book Hitler's Willing Executioners explains how millions of ordinary Germans were coerced into behaving immorally and against the ideals of Christianity. Look how evangelicals around trump have used each other to exploit conservative citizens to attain power and influence.
Your claim here assumes the people you're referring to are VERY self-aware, are self-reflective, and very conscious of their decision making. You are ignoring how we humans get lazy and complacent in how we think and believe. After 40 our brains move into a stage called Crystalization, and that means we naturally move into a static way of thinking. If we are not aware of this phase then we are not able to make better choices.
Why atheists keep trying to push this idea that religions brainwash people is puzzling given the fact that the atheist themselves are proof that it isn't happening.
This is a fair statement IF an atheist insists that it's brainwashing. The fact is there is an evolutionary and biological component to religious belief that is just our nature, the "wired for God" phenomenon.
But the kind of religion IS due to the influence of the community. This is why Islam is more prevalent in the Middle East. Or why Catholicism is more prevalent around Boston. Or why Southern Baptist is more prevalent in the American South. Or why Mormonism is more prevalent in Utah. or why being Hindu is more prevalent in India.
People learn the religion they are exposed to. The children don't choose for their parents and family to be Muslim, and teach them Islam, it is the circumstance of their birth.
This is false. Religions are not a geographical phenomenon. They are subcultures of their own and are no more tied to geography than any other culture is.
This is demonstrably untrue. There are ethnic food festivals for a reason. And how do you account for the list I wrote above?
I see no reason to eject these issues from this thread as they are all fundamental aspects of the culture of "atheism" in a social/debate context.
What culture of atheism?
Are you just referring to how many people are responding to atheism being more acceptable as an option in a highly religious nation?
Do you accept the freedom of thought the books written by atheists offer to thinking humans?
Irony overload, after this disjointed rant that simply ignores what I said and cries foul, just because I won't think and believe as you do. 
You are not content to hold your belief it seems, but must insist others do as well. 
