I started giving grit (sprinkling it like salt over their feed) when I began allowing "treats"....anything other than their regular formulated feed. I used fine parakeet grit when they were little. They should sell it for more mature birds at feed stores or tractor supply company...places like that.
If they'll be free ranging some when they're mature they probably won't even need it; they'll pick up small pebbles while ranging.
Since Chickens have no teeth they need gravel or Grit in their Craw to grind up the grain and bugs whatever they eat. I put starter grit in mine when they are small and like the teach said sprinkle it over the starter/grower like salt. As they get bigger they need larger grit. If they free range they can pick that up in the drive or where ever they go. I have grit for mine all the time and in the laying pen they have oyster shell all the time so they can get it when they need it
If you don't buy poultry grit, get some coarse-grain sand from your driveway or other source, and give to chicks anytime they are fed anything other than chick starter.
After 8-10 weeks of age they can use regular sized grit that is sold for chickens. MannaPro sells a 5lb bag and that's plenty to last a while. If you give them access to your driveway, or other sources of sandy, gritty, gravel-y dirt they will just get their own.
If they are outside they can find their own, but I offer it "just to be sure" - if your area gets a lot of snow or ice over the winter I would be extra sure to provide grit during times that they can't reach the ground.
Grit is small rocks and pebbles that are ingested and go to the gizzard, where they are used to grind up food.
Oyster shell is not grit, it is a calcium supplement for layin hens.
Most bagged feeds will tell you if you need to give some grit, most starters you dont have to . . .
My brand recommends grit once they start eating larger pellets of it . . .
If they get anything to eat besides a crumble feed, such as grass or scratch, they need grit.
No matter what age or size.
For small chicks most folks who buy get parakeet gravel, but a shovelfull of coarse dirt should have the right size grit for any sized chicken.
If they have access to outside ground, they have grit.
I would go so far as to say that if you keep your chicks on litter that can be eaten like pine shaving, you should give grit . . .
Put it this way, the right size grit will not hurt!