My bunch are mostly about a week older than yours and I've just run out of my last bag of starter. I'm gonna be getting grower now. I've got a mixed age flock and the roosters don't need the extra calcium, so I don't think I'll go to a layer when the oldest start laying. I intend to just keep feeding grower and supplement the calcium with free choice oyster shell.
Not sure if you need to add oyster shell if you are going to feed layer feed. I believe that is already supplemented with extra calcium. 'Course I doubt the oyster shell will hurt them if you did.
As for when to go to layer, you can do that when they get to an age that you should start expecting them to lay (that may depend on the breed but I think it's usually around 20 weeks give or take). Or just wait till you get your first egg, then you'll know for sure they'll be needing the extra calcium.