Thanks for this, I always wondered why my chickens never squat for me!
I have Sussexes (4 of them), not quite 16 weeks old, and one of them laid her first egg yesterday.
If you have chickens of different ages, some nowhere near laying age, just keep thrm all on the grower feed and, as others have said, put oyster shell in a separate container. Those who need it will help themselves, the young ones won't mess with it. You're also providing grit separately too, right? That's for digestion.
Go easy on the snacks and treats. Think of them as chips and ice cream. The more of those they eat, the less balanced nutrition they're getting. Enjoy your flock - and their eggs!