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!