It is not an age when they need grit. It is based on what you are feeding them. You can cut a chunck of your lawn with grass and dirst and let them have it. They will eat the grass and the dirt they eat will act as grit. Grit grinds the food as it passes through the gizzard. If you give them things other than chick feed they need some chick grit or dirt to go with it. Feeding commercial starter you don't extra grit.