Just to be clear about a couple things. Grit and oyster shell are 2 different things. Both are to be offered separately from the feed.
All ages of birds need insoluble grit to aid in grinding up their food.
Oyster shell is soluble and breaks down quickly in the acidic environment of a chicken's gut. Oyster shell is a superior source of calcium because of it's large particle size. Only laying hens need oyster shell.
I still wouldn't feed layer feed until the birds start laying. You don't know when a pullet will start laying especially this time of year. If a pullet turns 18 weeks in say November and doesn't start laying till days get longer in March and you started feeding layer in November, that's 5 months of excessive calcium they can't avoid.