Be careful, excessive calcium, before it is needed, can cause kidney and other issues and shorten their life. It is a very fine line to walk with mixed flocks.@BY Bob This is a really good point.
I have 6 adults and 2 babies, also 15 weeks old, so very similar to Bob's situation. We have been feeding the whole flock some regular layer pellets that is 17% protein since a month ago ...
In addition to what I said above, I keep littles with mom locked away first thing so they eat starter only, and have a creep so only the littles get the starter (i.e. so the big girls don't eat it on them.) as they get bigger and 'outgrow' the creep, they are transitioning to grower anyways, then I just fill all feeders with grower they can access, and some feeders only the bigs access with layer. eventually (as now except for my newest littles, 1 week & 2 weeks old) I'm using my own mix of 'all flock'