A Catholic baptized at birth can leave later without severe penalties, as they have not yet made a decision. Even after confirmation, they often do become inactive or change religions. It is rare that their whole lives fall apart just because they leave their church.
When the witlesses get baptized at 12, they are stuck. They cannot become inactive without Brother Hounder showing up. If they break one of the many ever-changing rules, they are in for houndings or cutting off by the religion. And they have to believe everything the Filthful and Disgraceful Slavebugger puts out without questioning it.
From what I hear, they are pushing even younger children to get baptized. I have heard of cases of children as young as 6 getting baptized, and they make a showcase out of it.