Breed is important to feeding needs.
You have commercial layers, although the Orpington, being dual purpose, will mature more slowly than the production reds (your feed store RIR) and production Barred Rocks, you still have feed store varieties which have been bred and selected for egg production.
I personally would not give any feed over 18% to at most 20% protein chick starter. 22% is for meat birds, and above that game birds or turkeys. You can harm growth by adding too much protein. The muscles can outstrip joints and bones causing malformations.  22% is meant for fast growing meat birds who need to add a lot of meat quickly. Game bird feed fed continually is not healthy for most commercial layer types, which you have. Show birds and heritage types may do better with higher protein, but I do not recommend that for commercial layer breeds.
Medicated is amprolium based which slows the uptake of vitamin B. It literally starves the coccidia protozoa by not allowing it to metabolize B. Medicated feed has its place in areas with feed store chicks who have not developed an immunity to coccidia or transplants who do not have an immunity to your coccidia strain. It is meant to be fed for about 10 weeks until the bird is mature enough for their own immune system to take over. Medicated feed merely slows the development of coccidia in the gut to allow that to happen.
Medicated feed will not hurt older hens, and you do not need to pull eggs. In time, though, steady consumption of the medicated may cause declined levels of B in your hens.
Since your older birds are commercial layers types, they will need calcium sooner as their genetics are geared for egg production sooner. You can actually delay the start of laying by having low calcium for the production breeds.
Chicks won't eat the oyster shell, so that is one method of accommodating all ages. Put out 18% chick start and leave oyster shell out for free feed. The older birds will begin to eat the oyster shell while the younger birds will not. You will know when point of lay is around the corner as the young pullets will begin to guzzle the oyster shell.
When your younger birds (shall I assume they are commercial laying types too, similar to your older birds?), become about 14 weeks of age, you can switch everyone onto a good layer. Studies have shown that the commercial laying breeds do best for production when switched at about 14 to 16 weeks of age rather than waiting for actual laying.
My experiences.
LofMc