My chicks are 9 weeks old. They have been going out to a grassy run for over a week. (Won't be long before the grass is gone.) They have had chick grit while in the coop because some of them were munching on shavings and small feathers. I have two bags of granite grit made in Mount Airy, NC. One bag is chick grit and the other is grower. At what age should I switch from chick to grower grit?