If you co-mingle the youngsters with the mature flock, you should feed grower/finisher or the equivalent of FlockRaiser and offer oyster shell free choice, as I do.
I keep the oyster shell in an empty tuna can nailed to an inside coop wall near a feeder. I also dump some in one particular spot in the yard because I have a laying duck who does not go into the chicken coop(s).
I tried feeding the different types to the two groups at first, because the mature birds were in one coop and the rest were in other coops (they all free range). The youngsters didn't get the laying feed, but the grown birds went for the grower/finisher feed a lot MORE than they ate the laying feed. When I got a couple of soft-shelled eggs, I realized I had to fix that.
So now I don't even buy laying feed. They're all on the same thing (except that chicks under 8 weeks of age are still on chick starter) and the crushed oyster shell is out there for the layers.