I'm a mother of three children and from my experience, there is no age limit. I believe if you show them religion as it is around them, they will develop their own questions and come to you as the parent for answers. My two oldest went to church ran preschools. I would take them to Sunday Church and put them in the classes with other children while I sat in the common area and read or played on my phone, lol. They went to VBS during the summers because their friends and cousins went. This was never forced on them in anyway. I didn't talk about not being a believer myself. My oldest stopped doing things like that around 7 but never asked me any questions. Just told me the Bible belongs in the fiction section. My middle child enjoyed it for a while. Than one Summer when she was 8 she didn't want to go to VBS, I asked her why and she said the bible was stupid and tries to make people feel bad. We talked about some of the things that bothered her and she just couldn't get over Cain and Abel's wife's. Where did they come from? She kept asking. You can't make billions of people with just two people. Why does nothing happen anymore like it did in the Bible? Anyway , you get the point. She hasn't gone back since. My youngest is only five and she really doesn't understand much right now, so we'll see. As long as you allow children to think for themselves their mind isn't clouded with crap.

Each child is different. I think they should wait to pick a religion where a baptism would take place until they are adults. Hopefully they choose to not believe in an invisible man in the sky.

This is a progression. Making proper choices comes as a result of their observing you, making mistakes and learning from them, and interacting with the world. It is very important that they receive guidance and acknowledgement when they do make proper decisions. As they grow older, they should make more and better decisions.

It depends on what the decision is about.