I don't think layer food has anything in it to MAKE the girls start laying. It just has extra calcium in it for the egg shells. Personally, my opinion, I don't think you have done anything wrong by feeding them layer food at the age they are. Seanb, have the girls started paying attention to the nest boxes? The roos may try to assist with "finding" a good place to nest, showing the girls how he thinks she ought to be kicking up the straw, turning around in the nest, etc. Cute to watch. Also, the girls' combs and wattles will get red, as they get ready to start laying. My White Rock pullet started laying at 16.5 weeks, so you may get lucky soon!