HELP WITH CHICKS

I bought 4 chicks a couple of days ago. Two of them look like they are a couple of weeks old. Two of them are probably around a week old. I bought some chick starter food. My questions are:

At what age do I need to feed them grit. The food does not specifically say it contains grit.

At what age can I feed them other stuff such as carrots, strawberries, apples, etc.

Is there anything I should NOT feed them?

I've heard milk (whether it is sour or not) is good for them, is this true?