My "chicks" are now 18 weeks old, but I have been giving them chick grit since they were 3-5 weeks old (I forget when I started), that's when they started getting "treats" of hard-boiled egg yolk, I moved them up to BOSS and Meal worms around 8 weeks. I have a small child's sandbox in my run for the girls to dust bathe in ($30 and shaped like a cute little turtle...designed for babies and holds 150 lbs of sand if filled to the brim), and they have a smallish pile of the chick grit (almost out of the chick grit, will be moving them up to the chicken sized stuff) on the ground. The run also has a dirt floor under the river rock that was previously put everywhere in my yard (I hate the stuff) that they dig around in all the time.
Oddly enough, they eat the sand most often to use as grit! I see them eat the dirt and they do an excellent job of tossing the grit around under their feeder (where I put the pile), but they go over to the sand box and grab a mouthful before breakfast every morning! It's funny to watch, they used to "sneak" it, but now they just walk over and act like they're getting a drink of water in the sand, lol.