At what age can a child decide on his own religion?

I was raised an atheist and I’m also raising an atheist family, well, until each person decides otherwise. I have a ten-year old boy and my little girl is 3 years old. We will never force our kids to go to church, because we don’t support religion of any kind. But if they should decide around their teenage years that they want to go through with a religious confirmation, then we definitely won’t deny them. It’s their life, their choice and when you’re 13 years old or so, you should be old enough know whether you’re religious or not. That doesn’t mean we do not mingle with theists, we have family members and friends who are Christians. And we try as much as possible to visit family members, follow them to church for a funeral, baptism, etc., we of course show up for those things out of respect for the families. Just because you’re visiting a church and don’t believe in anything said nor done in practice in there, you don’t turn into a moral-free asshole. Well, that’s at least what we try to teach our children, even though we’re atheists.