For starters I would take issue with the characterization of religion as “conservative and shame-producing religious doctrine.” It reveals the author’s bigotry towards people of faith. How is teaching someone that they are created in the image and likeness of the God who is love conservative or shame producing?
As for why children are taught religion rather than waiting until they get older, that is simply because parents always impart their values to their children. What parent would wait to until a child is old enough to make their own decisions to teach them that stealing is wrong? Or that bigotry and prejudice are wrong? Would the author in question have no problem with another child beating up his/her child under the explanation that the bully was simply exploring his moral options?
What parent doesn’t teach their children basic scientific facts about the world rather than waiting until they are old enough to do their own experiments?
Would the author let his/her child smoke cigarettes at a young age under the guise that its not the role of the parent to point out the harmful behavior?
The problem with the “wait until they are old enough to make their own decision on religion” argument is that it is never applied to any other value or issue of serious concern.