Until chicks are fed whole grains, they do not need grit fed extra. However, I do offer "baby grit", a fine-grind granite, when they go on pine shavings, as this can ensure that they will be able to get all that they eat through their tiny gizzards. The gizzard is able to mash up most solid thing that they eat as chicks, and also in adult birds, but for best utilisation of all whole feeds, granite grit is the most efficient. The gizzard will just break down sandstone and other "soft" stone or sand--and it must be renewed constantly--but that is all that is needed to utilize their feed. Granite remains in the gizzard for ages, as it does not wear down easily==and the chicken knows when it needs more. Having it offered free choice, the birds just take some when needed. I never mix it (or Oyster Shell, either) with their regular crumbles as they will be taking (or not taking) it as needed, but only because it is there--maybe more than necessary! (Are chickens that have granite grit healthier than the billions of chickens that do not??--I don't know--probably not?)
You can learn a lot about grit if you butcher and clean your own chickens. Few of us do this any more? The gizzards make the best chicken broth available!!! CJR