We give our hens layena from about 6 months old. When we had our younger pullets with them we had a separate feeder with start and grow. We figured all chickens ate from both feeders but hoped they would average out. The Layena has more calcium which I read was not so good for growing pullets but I didn't have room to separate them. Everyone turned out fine.
Wait until 7 mos? I've never heard that. Honestly, quite a few of my girls were laying before they hit the magic 5 mo mark. If it worries you, you could feed everyone flock raiser which is a step between chick starter and Layena and then just offer oyster shell for the ones who want it. But mine ate Layena and none of them have keeled over yet.