When does a 'child' become an 'adult' in Christianity?

[Oldiesfreak1854]

I would probably say when they can think and act wisely and maturely, and be able to understand basic Christian doctrines.

Interestingly enough though, whenever children are mentioned in the Bible, they're used as a symbol for salvation.  For example, when King David's first son with Bathsheba dies in infancy, he says that "he will not come to me, but I will go to him."  Granted, the ancient Hebrews didn't really believe in an afterlife, but this still suggests salvation to me.  Likewise, you have at least two occasions in which Jesus says to accept the Kingdom of Heaven like a child.  I come from an Anabaptist denomination that opposes baptism/communion for babies; we do, however, dedicate children to God.  Personally, I would be fine with baptizing a baby as a form of dedication, just as long as it makes clear that it is the choice of the parents to raise him/her as a Christian, and not the choice of the child to become a Christian themselves.  My uncle is a Lutheran minister, and he did that for me.  I was baptized again at my discretion when I was 13, though.