Whatever the legal age is in your area (usually 18). When still under the care of their parents, most children would probably not be allowed to decide to pursue a religion that is different than the parents' beliefs.
In West Virginia, a child can decide which parent they want to live with when they are 18.
It depends on the child's age and where he or she lives.
Like any religion - you learn about it and get to know people into it, weigh what you get out of it - and decide. You must renounce other faiths, in most cases. If you want to become a Jew, that varies with the type of synogogue. Most have classes, and elders to mentor you.
They have to be 18.
You have to be 18.
18 years
They have to be 18yo.
Islam
16
When they are 18.
You have to be 18.
When they are 18.