When I first was researching keeping birds I learned that you shouldn't feed layer to too young of bird. I know that one feed bag I used to buy recommended switching to layer at 18 weeks of age.
Say if you are starting out and all your babies reach 18 weeks and no one is laying I wouldn't start until 1/2 or more have started laying but provide oyster shell on the side. I have a very diverse flock of many ages. Some birds are over a year old, some are just starting to lay ~24 weeks of age, and a whole another group that are just 13 weeks. I've notice with my different groups coming into age that my birds (of several different breeds) all start laying around 22 weeks or more, not so much less.
Anyways, currently my whole flock is on start/grower crumbles and I feed oyster shell on the side. I plan to switch over to a layer feed around the end of November when my youngest group will be over 24 weeks and still offering oyster shell free choice of course. But I'm also considering just keeping the flock on the starter/grower throughout the winter and then switch over to layer in the spring. Last winter I had NO decrease in egg production what-so-ever (without supplying artificial light source), but figured for my 1+ year old birds that they may take a break this winter. If that is the case, I don't know if feeding layer would be the best choice for the non-layers. Anyways, hope my answer helped.
I've also had people tell me that I shouldn't be feeding my laying hens anything but layer, but I don't think they really understood that as long as you are supplying a calcium source that they will be alright.