There are as many opinions here about it as stars in the sky. Personally, I don't give grit until the chicks are big enough to go outside. ie. they are feathered and out of the brooder. All they need is a proper chick feed.
Once they are outside, then they get a grit mixed in with their feed (per instructions on the bag). At that point they are eating other things and need the grit to grind up their food.
You need to give them grit when you start feeding them anything other than chick feed. Last year I set out a container of grit at about 1 week old. I provided chick grit until they were about 12 weeks old. Then I switched to chicken grit.
I do not mix grit with feed. I have it available to them free choice 24 x 7 in it's own container.