Not sure of your situation, but you could:
-switch to a layer pellet that is pretty large and the young ones may not eat it because of its size. You will have to watch and make sure that it the case and
- construct a creep-feeder where the slats or holes are small enough the 2 month-olds can slip through and eat their crumbles but the laying hens are too big to fit through and the feed is placed far enough away they can't get their snake necks over to the feed pan
 
or
 
feed everybody the chick food and provide oyster shell in an elevated pan too high for the young ones to reach that the big girls to snack on