Oyster shell and grit are NOT the same thing.
Grit is insoluble rock particles that are stored in the gizzard to grind feed. They wear down with time and are passed so the birds will eat more as needed.
Oyster shell is a calcium supplement primarily used in mixed flocks where feeding layer feed is not recommended.
If your birds have access to the ground, they shouldn't need grit but it doesn't hurt to provide it. I keep some out year round and it is used more during the winter when the ground is frozen.
You can put out oyster shell when the birds are about 18 weeks old.